Actor Will Smith will host Washington's ultimate party of '99, the multimillion-dollar New Year's Eve celebration on the Mall for which a who's who of rock, soul and blues music will sing backup, organizers announced yesterday.

"Just imagine for a moment," first lady Hillary Rodham Clinton began at a news conference that provided few new specifics of the free, morning-to-midnight event but much lyrical description of its atmosphere and purpose -- "the music of our century rocking and rolling, swinging and soaring."

Some of those tunes will be belted out by Chuck Berry, Aretha Franklin, and B.B. King, who will perform during festivities that will stretch down the Mall and also for several blocks along Constitution Avenue.

Come evening, the spotlight will shift to the Lincoln Memorial, where Smith will lead a show to be produced by musician Quincy Jones and filmmaker George Stevens Jr. And in the century's final hour, as composer John Williams conducts an original orchestral score, an 18-minute movie by Steven Spielberg will highlight major events of the last 100 years before a high-tech sound and light display punctuates the first minute of 2000.

"It will be in the sky, and it will be inspiring," Stevens said yesterday.

"America's Millennium," as the family-friendly extravaganza is billed, is being organized by the White House Millennium Council, Smithsonian Institution, National Park Service and National Park Foundation. The city of Washington, which will mark its bicentennial as the nation's capital in 2000, is a partner in the planning.

The initial projection is for a turnout of 600,000 people. Only two gatherings on the Mall have been bigger: President Lyndon B. Johnson's 1965 inauguration and the 1976 bicentennial fireworks.

Already, the price tag has grown to $12.5 million from early estimates of $10 million. The total includes the lectures, demonstrations and performances that the Smithsonian will present from Dec. 31 through Jan. 2, but does not account for the city's companion block party along Constitution Avenue -- a "Main Street Millennium" expected to resemble a winter carnival of minstrels, magicians, acrobats and food tents between 10th and 14th streets NW. The city has not yet hired a contractor to run its portion.

The entire celebration will be privately funded, organizers reiterated yesterday, saying that with 95 days to go, half of the $12.5 million has been raised. Democratic Party fund-raising magnate Terry McAuliffe, who recently offered $1.3 million from his own accounts to guarantee a New York house mortgage for the first lady and President Clinton, is leading the money effort.

Despite some private concerns that the ambitious and time-pressed plans may exceed funds -- the Smithsonian, for instance, has yet to receive a budget for its role -- organizers were enthusiastic yesterday. One of the guests they introduced was their first million-dollar contributor, Omaha business executive Vinod Gupta. No other individual or corporate donors were identified, nor was any cost breakdown discussed.

"This is a tremendous opportunity to unite our nation, to capture an extraordinary moment in American and human history," said Hillary Clinton, who was joined at the news conference by Stevens and D.C. Mayor Anthony A. Williams (D).

With congressional representatives and local residents participating, the weekend's opening ceremony will take place near the Capitol as a time capsule is sealed with the reflections of national leaders in varied fields. Throughout the day on Dec. 31, Clinton said, the Smithsonian will offer a "digital time capsule" to which visitors can add their thoughts and wishes for future generations.

Producer Stevens, the man behind the annual Kennedy Center honors program and founder of the American Film Institute, gave no timetable for releasing a full schedule of performers. For the moment, the publicity boasts of bands, choirs and military marching units. Stevens acknowledged that many celebrities were booked long ago but said it was not too late to get commitments from others.

"It's a historic opportunity. You can perform in Las Vegas any night of the year," he laughed.

One lure would be the likely worldwide audience. Organizers said CBS will broadcast live from 10 p.m. through midnight, and arrangements to carry the show around the globe are pending. Spielberg's movie, "The Unfinished Journey," will be shown on two giant digital screens to be positioned on either side of the Lincoln Memorial.

Although the Mall will return to normal Jan. 3, the city's bicentennial commemoration will just be starting. James V. Kimsey, chairman emeritus of America Online, is leading the steering committee of city and regional leaders that will plan a year of programs and festivities and raise the private funds to pay for them.

Mayor Williams's special assistant, Sandy McCall, said the calendar of downtown and neighborhood events will not be publicized until January. Kimsey's committee members, however, will be announced by early November.

"When [others] see this new board," McCall predicted, "they'll see there's been an immense vote of confidence in this new mayor and the renaissance of this city."