There was a strong theme of unfinished business, with speakers lamenting new attacks on civil rights.
"This is not the time for nostalgic commemoration," said Martin Luther King III, the eldest son of the slain civil rights leader.
"Nor is this the time for self-congratulatory celebration. The task is not done. The journey is not complete. We can and we must do more."
Eric Holder, the first African-American attorney general, thanked those who marched a half century earlier.
He said he would not be in office, nor would Mr Obama be president, without them.
"They marched in spite of animosity, oppression and brutality because they believed in the greatness of what this nation could become and despaired of the founding promises not kept."
The spirit of the 1963 march now demands equality for gay people, Latinos, women, the disabled and others, he said
Those in attendance represented causes advocating gay rights, organised labour and voting rights.
Other speakers cited unemployment among African-Americans, which is double that of white citizens, and the fatal shooting of unarmed black Florida teenager Trayvon Martin.
Mr Martin's killer, George Zimmerman, was acquitted after a jury found he had acted in self-defence.