"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.