Tens of thousands of civil rights marchers tried to recapture the spirit of Martin Luther King on the 25th anniversary of his "I Have a Dream" speech Saturday with warnings that the struggle to fulfill King's dream of equality is far from over.

Singing "We Shall Overcome," demonstrators led by Jesse Jackson, Democratic presidential candidate Michael Dukakis and Coretta Scott King, widow of the slain civil rights leader, marched from the Washington Monument to the Lincoln Memorial waving signs and banners in the hot midday sun.U.S. Park Police estimated the crowd at 55,000 people, far fewer than the 250,000 who thronged the National Mall for King's famous oration climaxing the march on Washington on Aug. 28, 1963, a crowd that was equaled five years ago at a 20th anniversary march.

Dukakis' appeal to banish all forms of racism and discrimination was greeted warmly, but it was Jackson - his former rival for the Democratic nomination - who got the biggest cheers of the day with a scathing attack on Republican nominee George Bush.

"George Bush is not in Washington today," Jackson said to a roar of approval. "He must not be here for the inauguration next January."

The vice president, citing previous commitments, did send a message to the marchers: "Your presence here today demonstrates that the spirit and dream of the late Dr. Martin Luther King remains alive."

Jackson said that a quarter-century ago, "we looked forward to new frontiers" under President Kennedy.

"Today, Reagan and Bush attempt to return us to an old fortress," he said. "They are trying to rebuild old walls. They have fought to crush the dream."

Jackson made no mention of Dukakis or the Democratic ticket, but he appealed for his predominantly black audience to vote in the Nov. 8 elections.

"Hands that once picked cotton can now pick mayors, legislators, governors and presidents," he declared.

Others at the rally echoed Jackson's message.

"Twenty-five years later, we are here stronger and more determined than ever," said Rep. John Conyers, D-Mich.

Conyers told the demonstrators they were not marching so Jackson could be the next president "but so that your son or daughter can be a future president of the United States."

Ethel D. Smith of Washington, who had marched in 1963, brought her grandson to the demonstration Saturday. Looking back, she said, "A whole lot of doors have been opened, but we've got a long way still to go."

Dukakis and Jackson met on the front ranks of the march for the first time since the Massachusetts governor defeated the civil rights activist for the Democratic nomination at last month's party convention in Atlanta.

President Reagan, spending the day campaigning for Republican candidates in California, issued a statement asserting that since King led the landmark March on Washington in 1963, "America has made vast progress toward fully achieving Dr. King's dream of a color-blind society."

As a result, Reagan said, "America today is a freer land for Americans of all backgrounds."

He added: "On this anniversary, it is fitting that all Americans should give thanks for that progress and for the work of those who sacrificed so much to bring it about. And let us remember, as well, that freedom is our unending challenge and our continuing vocation as Americans."

But Rep. Charles Rangel, D-N.Y., said the marchers are still "looking for jobs, looking for peace and looking for freedom."

Referring to a campaign dispute between Dukakis and Bush, Rangel said freedom is not about reciting the Pledge of Allegiance. Rather, he said, "freedom means our children can get an education as easily as they can get cocaine in our schools."

At the time, the 1963 demonstration was the biggest ever held in Washington. The peaceful, orderly march _ and King's galvanizing speech _ are credited with spurring Congress to pass the landmark Civil Rights Act of 1964 under pressure from President Johnson.

Mrs. King said the 1963 march "helped break the spine of racial segregation" in the United States, and that today "we still have a dream of a nation free of the cancer of racism."

The Rev. Joseph E. Lowery, president of the Southern Christian Leadership Conference, recalled that King had said, "America has given the Negro people a bad check, a check which has come back marked `insufficient funds'."

Lowery said the marchers had returned to cash that check.

"It's not that there are no funds," he said. "It's that your ways of accounting are faulty. We know the money is there. If you don't cash it now, we will be back again and again and again."