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“To dismiss the magnitude of this progress, to suggest, as some sometimes do, that little has changed — that dishonors the courage and the sacrifice of those who paid the price to march in those years,” Mr. Obama said. “But we would dishonor those heroes as well to suggest that the work of this nation is somehow complete.”

Ms. Winfrey appeared to inadvertently step into the controversy over a quote on the nearby King memorial when she said of the late civil rights leader, “He was, after all, a drum major for justice.”

The memorial originally was inscribed with a quote from King that read: “I was a drum major for justice, peace and righteousness.” Critics said it was a paraphrase that made King sound egotistical, and the phrase was removed from the monument at great cost.

King said, “Yes, if you want to say that I was a drum major, say that I was a drum major for justice. Say that I was a drum major for peace. I was a drum major for righteousness. And all of the other things will not matter.”

Rep. John Lewis, a Georgia Democrat who spoke at the event in 1963, recalled the joyous optimism of that rally and the greatness of King.

“Not one incident of violence was reported that day,” Mr. Lewis said. “The spirit of Dr. King’s words captured the hearts of people, not just in America, but around the world. He changed us forever. Fifty years later, we can ride anywhere we want to ride, stay anywhere we want to stay.”

But he said there are still “invisible signs” of racism.

King’s sister, Christine King Farris, 85, said she was “awestruck” by her brother’s speech in 1963.

“On that day, Martin achieved greatness,” she said, urging the crowd to carry forward his work. “Yes, they can slay the dreamer, but no, they cannot destroy his immortal dream.”

She, too, spoke of Trayvon Martin and voting rights as rallying points for civil rights advocates.

“We are not going to be defeated,” she said.

Before Mr. Obama spoke, he and others near the podium watched as members of the King family rang the bell from the 16th Street Baptist Church in Birmingham, Ala., where four girls were killed in a bombing in September 1963.