WASHINGTON » Standing on hallowed ground of the civil rights movement, President Barack Obama challenged new generations Wednesday to seize the cause of racial equality and honor the "glorious patriots" who marched a half century ago to the very steps from which Rev. Martin Luther King spoke during the March on Washington.

In a moment rich with history and symbolism, tens of thousands of Americans of all backgrounds and colors thronged to the National Mall to join the nation’s first black president and civil rights pioneers in marking the 50th anniversary of King’s "I Have a Dream" speech. Obama urged each of them to become a modern-day marcher for economic justice and racial harmony.

"The arc of the moral universe may bend toward justice but it doesn’t bend on its own," Obama said, in an allusion to King’s own message.

His speech was the culmination of daylong celebration of King’s legacy that began with marchers walking the streets of Washington behind a replica of the transit bus that Rosa Parks once rode when she refused to give up her seat to a white man.

At precisely 3 p.m., members of the King family tolled a bell to echo King’s call 50 years earlier to "let freedom ring." It was the same bell that once hung in the 16th Street Baptist Church in Birmingham, Ala., where four black girls were killed when a bomb planted by a white supremacist exploded in 1963.

Georgia Rep. John Lewis, a former freedom rider and the sole survivor of the main organizers of the 1963 march, recounted the civil rights struggles of his youth and exhorted American to "keep the faith and keep our eyes on the prize."

The throngs assembled in soggy weather at the Lincoln Memorial, where King, with soaring, rhythmic oratory and a steely countenance, had pleaded with Americans to come together to stomp out racism and create a land of opportunity for all.

White and black, they came this time to recall history — and live it.

"My parents did their fair share and I feel like we have to keep the fight alive," said Frantz Walker, a honey salesman from Baltimore who is black. "This is hands-on history."

Kevin Keefe, a Navy lawyer who is white, said he still tears up when he hears King’s speech.

"What happened 50 years ago was huge," he said, adding that there’s still progress to be made on economic inequality and other problems.

Two former presidents, Bill Clinton and Jimmy Carter, spoke of King’s legacy — and of problems still to overcome.

"This march, and that speech, changed America," Clinton declared, remembering the impact on the world and himself as a young man. "They opened minds, they melted hearts and they moved millions — including a 17-year-old boy watching alone in his home in Arkansas."

Carter said King’s efforts had helped not just black Americans, but "In truth, he helped to free all people."

Still, Carter listed a string of current events that he said would have spurred King to action in this day, including the proliferation of guns and stand-your-ground laws, a Supreme Court ruling striking down parts of the Voting Rights Act, and high rates of joblessness among blacks.

Oprah Winfrey, leading the celebrity contingent, recalled watching the march as a 9-year-old girl and wishing she could be there to see a young man who "was able to force an entire country to wake up, to look at itself and to eventually change."

"It’s an opportunity today to recall where we once were in this nation," she said.

Obama used his address to pay tribute to the marchers of 1963 and that era — the maids, laborers, students and more who came from ordinary ranks to engage "on the battlefield of justice" — and he implored Americans not to dismiss what they accomplished.

"To dismiss the magnitude of this progress, to suggest — as some sometimes do — that little has changed, that dishonors the courage, the sacrifice, of those who paid the price to march in those years," Obama said.

"Their victory great. But we would dishonor those heroes as well to suggest that the work of this nation is somehow complete."

Civil rights activist Myrlie Evers-Williams, whose husband Medgar Evers was murdered in 1963, said that while the country "has certainly taken a turn backwards" on civil rights she was energized to move ahead and exhorted others to step forward as well.

King’s eldest son, Martin Luther King III, just 5 when his father spoke at the Mall, spoke of a dream "not yet realized" in full.

"We’ve got a lot of work to do but none of us should be any ways tired," he said. "Why? Because we’ve come much too far from where we started."

Organizers of the rally broadened the focus well beyond racial issues, bringing speakers forward to address the environment, gay rights, the challenges facing the disabled and more. The performers, too, were an eclectic crowd, ranging from Maori haka dancers to LeAnn Rimes singing "Amazing Grace."

Jamie Foxx tried to fire up a new generation of performers and ordinary "young folks" by drawing on the example of Harry Belafonte, who stood with King 50 years ago.

"It’s time for us to stand up now and renew this dream," Foxx declared.

Forest Whitaker told the crowd it was their "moment to join those silent heroes of the past."

"You now have the responsibility to carry the torch."

Slate gray skies gave way to sunshine briefly peeking from the clouds as the "Let Freedom Ring" commemoration unfolded. After that, an intermittent rain.

Obama spoke with a bit of a finger-wag at times, saying that "if we’re honest with ourselves, we’ll admit that during the course of 50 years, there were times when some of us claiming to push for change lost our way." He spoke of "self-defeating riots," recriminations, times when "the bigotry of others was reason to give up on yourself."

But the president said that though progress stalled at times, "the good news is, just as was true in 1963, we now have a choice."

"We can continue down our current path, in which the gears of this great democracy grind to a halt and our children accept a life of lower expectations; where politics is a zero-sum game where a few do very well while struggling families of every race fight over a shrinking economic pie — that’s one path. Or we can have the courage to change."

Among faces in the crowd: lawyer Ollie Cantos of Arlington, Va., there with his 14-year-old triplets Leo, Nick and Steven. All four are blind, and they moved through the crowd with their hands on each other’s shoulders, in a makeshift train.

Cantos, who is Filipino, said he brought his sons to help teach them the continuing fight for civil rights.

"The disability rights movement that I’m a part of, that I dedicate my life to, is actually an extension of the original civil rights movement," said Cantos. "I wanted to do everything I can to school the boys in the ways of the civil rights movement and not just generally but how it affects them personally."

D.C. plumber Jerome Williams, whose family tree includes North Carolina sharecroppers, took the day off work to come with his wife and two kids. "It’s a history lesson that they can take with them for the rest of their lives," he said.

It seemed to work. His son Jalen, marking his 17th birthday, said: "I’m learning the history and the stories from my dad. I do appreciate what I do have now."

Performers included Peter Yarrow and Noel Paul Stookey, their voices thinner now than when they performed at the original march as part of the folk trio Peter, Paul and Mary. They sang "Blowin’ in the Wind," as the parents of slain black teenager Trayvon Martin joined them on stage and sang along. The third member of the trio, Mary Travers, died in 2009.

Also joining the day’s events were Lynda Bird Johnson Robb, daughter of Lyndon Johnson, the president who signed the 1964 Civil Rights Act and the 1965 Voting Rights Act, and Caroline Kennedy, daughter of John F. Kennedy.

Former President George W. Bush didn’t attend, but said in a statement, Obama’s presidency is a story that reflects "the promise of America" and "will help us honor the man who inspired millions to redeem that promise." A spokesman said the former president declined to attend because he was recovering from a recent heart procedure.

Associated Press writers Darlene Superville, Brett Zongker and Andrew Miga contributed to this report.