Twenty years after Dr. Martin Luther King Jr. aroused a crowd and a nation with his "I Have a Dream" speech for racial equality, tens of thousands of people will march here again this Saturday in a commemorative gathering intended to protect past gains and stir up support for new objectives not then envisioned by most civil rights crusaders.

The 20th anniversary of the historic March on Washington, billed as "March on Washington II," is attracting participants from more than 300 cities around the nation and will feature well-known civil rights activists and other speakers, including King's widow, Coretta Scott King, Gloria Steinem, Atlanta Mayor Andrew Young and the Rev. Jesse Jackson along with celebrity entertainers, such as Stevie Wonder, Bill Cosby and Peter, Paul and Mary.

March organizers are predicting a turnout comparable to the 250,000 who jammed the Mall in 1963. Except for use of the subway system, the program planning, logistical preparations and even the steamy August climate are supposed to be virtual replays of what occurred 20 years ago.

But March on Washington II will bring a much broader range of grievances and issues to the Lincoln Memorial. And in trying to rekindle some of the moral outrage, drama and hope that characterized the 1963 event, the marchers will be demonstrating just how far the country has come--and how far they think it still must go.

The original march was played out against a volatile backdrop of "freedom rides," sit-ins and highly publicized violence against civil rights demonstrators in the South, and it was staged primarily on behalf of sweeping civil rights legislation, then pending in Congress, to outlaw segregation and other forms of racial discrimination.

At the time, the nation's capital had never seen such a mass demonstration, and city and federal officials were uneasy hosts for the event. But the march was carried out peacefully, marked the pivotal point in winning the support of white moderates and propelled King into the forefront of the civil rights movement.

Saturday's crowd--with its peace, antinuclear, environmental, labor, women's rights, gay rights, handicapped, elderly and various ethnic-rights contingents--will be gathering to push a plethora of domestic and foreign policy concerns that so far have failed to inspire the same kind of sustained and popular passion. The groups involved, many of which did not even exist 20 years ago, also will be advocating a diverse array of legislative measures.

"We are not having just a civil rights march here in 1983, any more than we had one in 1963," says D.C. Del. Walter E. Fauntroy, the march's national director, explaining why the mobilization's expanded theme of "Jobs, Peace and Freedom" has attracted so many divergent supporters. Nor is the event supposed to "recreate a nostalgic trip down memory lane," according to another march coordinator.

The idea this time is to take the fight for civil rights and other issues of the day "the next step further."

With the Civil Rights Act long since enacted, march organizers have formed a "New Coalition of Conscience" and have agreed to work for approval of a bill to make King's birthday a national holiday and for passage of the Equal Rights Amendment banning discrimination on the basis of sex.

They are also supporting amendments to the Fair Housing Act, approval of the Hawkins Community Renewal Employment Act to create state and local jobs, and the Economic Equity Act, which would guarantee parity for women in pension, tax, insurance and child-care matters.

Additionally, they are opposing a bill that would impose sanctions on employers who hire undocumented alien workers.

In the area of foreign policy, march organizers support legislation that would cut defense spending, freeze the nuclear arms race, prohibit the export of nuclear material and technology to South Africa, oppose International Monetary Fund assistance to South Africa country and block new investments there by U.S. firms.

They also back a resolution calling for unconditional negotiations toward a peaceful solution of the conflict in El Salvador.

This broadening of the march's focus, however, and the attempt to keep its "very shaky" coalition intact, according to a leader of both marches, has caused controversy.

Several prominent Jewish groups, for instance, held off endorsing the march because of sections of the march's call and position papers that they interpreted as attacking Israel.

And no sooner had march leaders quelled most of this controversy, agreeing to keep their Middle East statements short and not too specific, than homosexual activists here and elsewhere learned there would be no speaker at the march to represent gay-rights concerns. The rationale was that including a gay speaker might give the appearance of advocating the gay life style, which some members of the coalition would find objectionable.

This attitude sparked a peaceful, 2 1/2-hour sit-in at Fauntroy's office yesterday by a group of gays, as well as protests from several gay-rights groups. Police said four persons were arrested in the sit-in and charged with unlawful entry.

Despite such last-minute conflicts, though, the marchers, including gays, will begin assembling on the Mall at 14th Street as early as 8 a.m.

There will be a pre-march program here between 9 a.m. and 10:30 a.m., when marchers will start the short trek to the Lincoln Memorial. The official march program is scheduled to get underway at 1 p.m.

In addition, some 30 separate "affinity" groups, representing the District's eight wards, numerous churches, labor, gays, students and other subgroups, will hold their own individual "feeder" marches through the city and to the Mall Saturday morning, beginning at 8 a.m.

Coordinating arrangements for the marchers has been every bit the mammoth assignment it was in 1963. Traffic is being sealed off for a 10-block area around the Mall. Other streets are being closed to take the parking overflow.

Some 4,100 out-of-town buses are being directed to fringe parking sites at various Metro stations and military bases. The subway will be operating under a rush-hour schedule all day, charging a 50-cent fare for all passengers.

At the Mall itself, 7,000 volunteer marshals will be on hand for crowd control. Water fountains and tanks will dot the landscape, and 17 first-aid stations will be set up at strategic points between the Lincoln Memorial and the Capitol.

About 950 portable toilets, including some specially equipped for the handicapped, are being trucked in.

There are some differences that are making march preparations run more smoothly this time around, according to Frank Hollis, a District official who is coordinating logistics for the gathering.

"Twenty years ago, there were no black sympathizers in positions of power, and we were sort of doing this for the first time and had to put it together in eight weeks," he recalls. "This time, the planning started two years ago, and we have the full support of the Park Service, D.C. police and the city government. It helps."

But other changes are less encouraging, march organizers say.

Pointing to the three-part theme of "Jobs, Peace and Freedom," they note that unemployment is almost twice what it was in 1963 and that minority unemployment has climbed from 11 percent then to 17 percent today.

Hard-won civil rights freedoms, they complain, have come to a standstill and are starting to be reversed in some case under the Reagan administration. Hate/violence activities are increasing, and many Americans still suffer from hunger and extreme poverty.

And while Saturday's marchers will come chanting for peace, march organizers say the number of wars worldwide has jumped from an estimated 15 in 1963 to 40 currently.