Tomorrow's Live Aid benefit concert is the biggest concert ever -- 63 superstar rock acts playing in two cities to raise tens of millions of dollars for African famine relief.

It is so big that the surviving Beatles may reunite with Julian Lennon, son of slain Beatle John Lennon, for grand finales at both Philadelphia's JFK Stadium and London's Wembley Stadium. Paul McCartney, Ringo Starr and George Harrison were reportedly rehearsing yesterday and today in London for what would be their first public performance together since 1968.

Several insiders with the Live Aid production, who asked not to be named, said the London show would end with "Let It Be," backed by a chorus of British rock stars. "If it happens in England," one source said, "the quartet will hop on a Concorde, fly to Philadelphia and repeat the performance here." A spokesman for McCartney in London denied that a reunion would take place, but confirmed that the mass finale would include "Let It Be" and "Feed the World."

Irish rock musician Bob Geldof, who kicked off the rock-against-famine movement last December with the British all-star group Band Aid's "Do They Know It's Christmas?," is calling Live Aid a global jukebox, but it will be producing money as well as music -- as much as $50 million in a single day, from ticket sales ($6.5 million), corporate sponsorships ($5 million), sale of broadcast rights ($7 million), merchandising ($2 million) and telethons in more than 20 countries where viewers will be encouraged to phone or write in pledges. It will also attract one of the largest television audiences in history, an estimated 1.7 billion people in 140 countries around the world.

"The good thing about the Band Aid projects -- I include USA for Africa and all the global things -- is that every event has to be bigger to attract people," says Geldof. "It's the glamor. It's an easy way for people to help."

According to the singer, money raised from Live Aid will be distributed to an alliance of 110 volunteer agencies to provide shelter, clothing and "long-term solutions" in Africa. So far, funds from Band Aid and USA for Africa, the American group that put out the hit charity record "We Are the World," have been used primarily to provide food and medical supplies.

In America it will be the Woodstock of the MTV generation. In fact, television viewers will probably have a better view of what is happening than the 90,000 fans in JFK Stadium or the 72,000 at Wembley.

"It's a first on a number of levels," says Worldwide Sports and Entertainment president Michael Mitchell, who headed finance and planning for the Los Angeles Olympics and is the executive producer of the Live Aid concerts. "It's the largest television show in history, certainly the longest, a 17-hour running show, and most countries in the world are carrying it for the entire length of time. There are some 60 major acts, which makes it in every sense the largest rock concert in history. It's also the world's largest telethon.

"But that isn't the point or the goal of the project, which is to have the entire world do something at the same time. Television's never been used this way, to create a catalyst of action, to create a day of such emotional interest and intensity that this question of hunger is actually faced on a worldwide basis . . .

"We are engaging people for a purpose."

Building on an unprecedented level of cooperation between television and radio networks, and the volunteered services of hundreds of musicians, Live Aid may not become the cultural landmark that Woodstock did, but it is certainly a moral high-water mark in rock's short history.

Obviously, the Beatles reunion is the most intriguing Live Aid possibility. For now Paul McCartney is scheduled to play with two members of Queen, in his first British concert appearance in seven years. The Philadelphia finale was originally to feature Bob Dylan leading an all-star sing-along of his '60s anthem "Blowin' in the Wind," with various stars singing individual lines a la "We Are the World." Besides Dylan, only four of the USA for Africa artists will appear in Philadelphia -- Kenny Loggins, Tina Turner and Daryl Hall and John Oates -- fueling speculation of a rift between the Band Aid and USA camps.

Ironically, Harry Belafonte's original idea for USA for Africa (inspired by Band Aid) had been to stage a superstar benefit concert, but he and project coordinator Ken Kragen rejected that idea in light of the financial failure of 1972's Concert for Bangla Desh and the US Festivals in California. During the decade that $12 million in Bangladesh benefit funds were tied up in litigation, 8 million children in Bangladesh died of starvation, according to UNICEF figures. The moneys raised so far by Band Aid and USA for Africa, some $55 million, have been very publicly accounted for.

"There is a natural competition" between Live Aid and USA for Africa, Mike Mitchell concedes. "But I truly hope Ken Kragen comes back and does something even more spectacular and raises another $500 million. We're by no means enemies."

Some healing will be evident at the concerts. Despite an acrimonious split several years ago, the Who have gotten back together, and Ozzy Osbourne will be reunited with Black Sabbath. Guitarist Jimmy Page and vocalist Robert Plant will perform some Led Zeppelin songs for the first time since the death of the group's drummer, John Bonham, in 1980. And Teddy Pendergrass, who has not performed publicly since a paralyzing car accident in 1980, will join Ashford and Simpson.

The Pretenders and Phil Collins will play both concerts, thanks to the Concorde. There will also be some strange on-stage combinations, including "Miami Vice's" Don Johnson singing with Duran Duran. Many of the stars will not be playing with their regular backup musicians (a smart move that should eliminate lengthy equipment change delays) -- some will be thrust into instant supergroups and some will perform solo. In England, Collins will team up with Sting and Branford Marsalis. Elton John will play piano for the duo Wham!.

A transatlantic duet by Mick Jagger and David Bowie has been scrapped. "It created impossible problems," says Mitchell. "There was a four-second delay because the signal had to go up and down and through the ground stations. We practiced it once and it was intolerable." Instead, Jagger and Bowie made a video of the '60s Motown hit "Dancing in the Streets." (A single will be released next week, with all proceeds going to Live Aid.) Jagger will now do a live duet with Tina Turner in Philadephia, although a Rod Stewart-Madonna duet has been scrapped because they couldn't agree on a song.

Each set was originally scheduled to last 22 minutes, but as the roster grew to epic proportions, particularly in Philadelphia, cuts started being made. "We're trying to accommodate a lot of people," says Mitchell. "We've shrunk several sets down to 10 minutes. A lot of people who've never sung together are going to sing together, just so they can be involved."

Scenes from smaller concerts around the globe will appear on U.S. television screens via satellite. In Australia, Men at Work, INXS, the Little River Band and more will perform at the Sydney Entertainment Centre. Other bands will take the stage in Japan, Germany and Holland (B.B. King).

And in something of a coup, the Soviet hard rock group Autograph will perform live from Moscow, becoming the first Soviet rock musicians to be seen by a worldwide television audience, according to Brian Bedol of Dalrymple and Bedol Communications in New York, which will produce the concert in Moscow.

Virtually unknown in America but quite popular in the Soviet Union, Autograph will perform in a television studio in Moscow before an audience of about 1,000. In Philadelphia the performance is scheduled for 10:58 a.m., following a duet by Phil Collins and Sting that will be broadcast from London. Concertgoers will see the group on giant video screens. Technicians are trying to fashion a split-screen effect, Bedol said, so one side will show Autograph and the other side will show American and British audiences watching.

As a fund-raising concert, Live Aid is notable enough. As a television event, it is unprecedented.

"We've not only tried to reach most of the people in the world with this show, but everybody in the world," says Mike Mitchell. "Several countries don't have satellite downlink capabilities -- about 40 out of 164 -- but we just cut a deal to get them the tapes within six hours through diplomatic pouch."

In reaching an estimated 500 million out of 600 million homes with TVs, Worldwide could break the record viewership of 1.5 billion -- set by the 1982 World Soccer Cup. But those figures may be skewed. Two of the largest television markets, the Soviet Union and China, now say they will not be carrying Live Aid live, although the Soviet Union has agreed to take the satellite feed to be rebroadcast later, according to Brian Bedol. (It won't be the first time the Soviets will see a live rock concert from America. In 1982 and 1983 the United States managed a satellite hookup with the Soviet Union to televise the US Festival from Los Angeles.)

The challenge of broadcasting Live Aid is tremendous. "We're using 16 satellites," Mitchell says. "The Olympics used three . . . We're taking shows from seven countries, bringing them in here, putting them into one show, putting them back out, and we're producing four feeds at once -- for MTV, for the world, for the independents and for ABC."

ABC will be using 14 cameras and the latest technological wrinkle, the Skycam. Allowances are being made for the world and MTV feeds, while ABC has the rights to six taped performances from earlier in the day. "ABC clicks on at 8 p.m. and clicks off at 11 p.m.," says Mitchell. "If it runs over, certain things won't happen on ABC. Everybody is sensitive to running this as a timed concert."

Many, however, are privately conceding that it probably will run over. There will be some rehearsing today, says Mitchell, "just a walk-through for some people who can get there, some skits and timing. But by and large, what you see is what you get. At 7 a.m. we're going on the air, and we're all going to be scrambling."

And the show will go on (and on, and on) come rain or shine.

"The stage at JFK will be covered, and so will the TV cameras," said ABC production manager Vince Scarza. "The technical people working on this show have operated in conditions far worse than you could imagine. And if it blowed up so hard that the acts couldn't go on stage, we'd simply switch the show to other locations."

Music is not the only thing Live Aid will broadcast. Television audiences will not see famine pictures from Ethiopia or Sudan, but will get hourly "inspirational messages" from assorted political, religious, scientific and sports figures, including former president Jimmy Carter and ex-candidate Geraldine Ferraro, Nobel Peace Prize winner Bishop Desmond Tutu, Tanzanian President Julius Nyerere, Indian President Rajiv Gandhi, Sen. Edward M. Kennedy (D-Mass.) and Rep. Jack Kemp (R-N.Y.).

Scientists Linus Pauling and Carl Sagan, actors Kirk Douglas, Burt Lancaster and Sally Field, Soviet poet Yevgeny Yevtushenko, sports stars Pele and Kareem Abdul-Jabbar, Sirs Richard Attenborough and Edmund Hillary and others will urge people to make donations. There may also be taped messages from President Reagan, Pope John Paul II and British Prime Minister Margaret Thatcher.

Jihan Sadat and Coretta Scott King, the widows of assassinated Egyptian President Anwar Sadat and slain civil rights leader Martin Luther King Jr., have also videotaped messages, and earlier this week Mike Mitchell was still "trying to arrange some live interactive stuff with several leaders of the world. Saudi Prince Sultan, who just went up in the space shuttle and made the statement about no boundaries in space, just did a piece for us."

The telethon pitches will appear every hour as three-minute inserts, "an inspirational 45 seconds, an educational 45 seconds and a 45-second appeal for the fund," Mitchell says. "You know, there has never been an international telethon before, and we may end up with telethons in 30 countries," including Canada, Australia, Britain, Japan and New Zealand.

"If each one produced just a million dollars -- which is nothing in the United States -- well, you see the potential for $40 to $50 million."

Live Aid has also arranged several million dollars' worth of corporate sponsorship, similar to the Olympics. "The corporations in America are a key element in solving this problem," Mitchell says. "But to be very truthful, it's a double-edged sword for them. As they get involved with us, there's a great deal of pressure from other causes to say why weren't you involved with us also? So it hasn't been an easy effort, but we have a good group of sponsors." They are AT&T, Kodak, Chevrolet and Pepsi.

According to Mitchell, the corporate sponsorship is "symbolic, because it's government, corporations and people that will solve the problem of hunger. Alone, they can't do that, so the show has attempted to marry all three forces."

Despite its good intentions, Live Aid has not been without its critics. Some have grumbled that Philadelphia's offering JFK Stadium rent free (the usual rate is $100,000 a day) is a public relations ploy designed to divert attention from the recent bombing of MOVE headquarters that left 11 dead and hundreds homeless. In that light, Mayor Wilson Goode's statement that Live Aid would "underscore our status as the city of brotherly love" seemed particularly self-serving.

Others have complained that the "rock celebrities against famine" syndrome represents not conscience but calculated career moves. There's no denying that a shot at Live Aid's worldwide audiences would provide immense promotional value. It's also likely that following the excessive publicity for the "We Are the World" sessions, no musician in his right mind would want to be left out in the cold.

"Somebody asked me a few weeks ago how we kept Live Aid quiet so long," says Mike Mitchell. "This was a week after the press conference, and I said that for months I'd knocked on the door of every ad agency to explain this project and I couldn't get anybody to pay attention to me. In fact, it has gathered a great deal of interest and momentum, and we have a lot of people who now want to be involved." While there was some jockeying for position, he adds, "by and large, 99 percent of the acts know why they're there and are totally cooperative."

The major complaint against Live Aid has been that it excluded black artists (some thought deliberately) in favor of more commercial white acts, particularly those championed on MTV, and that it also excluded low-income fans who couldn't afford $35 tickets. But Geldof points out that Live Aid is all about raising money, that all of its performers are working for free and paying their own expenses, that all media are paying for their tickets. (Concert expenses will be more than covered by the several million dollars in rights fees paid by ABC as well as West German and Japanese networks.)

When the shows were first announced June 10, Geldof and American promoter Bill Graham insisted that they had asked "every major black artist" to participate, including Michael Jackson, Prince, Lionel Richie and Ray Charles. Of the 48 acts originally scheduled in London and Philadelphia, the only black artists were David Ruffin and Eddie Kendricks, Billy Ocean, Sade, Joe Leeway of the Thompson Twins and Tony Thompson of Power Station.

"For six months I've been trying, and the end result is what you see," Geldof said, adding that he had tried to approach major black artists through their managers, record companies, personal friends, video directors and so on. "We invited everyone. If someone isn't there, draw your own conclusions."