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 Jr. 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., before the church was bombed in 1963.

Georgia’s John Lewis, a Freedom Rider-turned-congressman, 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.

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.

Organizers of the rally broadened the focus well beyond racial issues, bringing speakers forward to address the environment, gay rights, challenges facing the disabled and more. The performers ranged from Maori haka dancers to LeAnn Rimes singing “Amazing Grace.”

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.”