President Obama, paying tribute to Martin Luther King Jr. exactly a half-century to the minute of King’s resounding plea for racial justice, said those who sacrificed their lives for civil rights decades ago did not die in vain because the country has made great strides toward equality, but he also called on Americans to “keep on marching” to fulfill King’s dream.

“We rightly and best remember Dr. King’s soaring oratory that day, how he gave mighty voice to the quiet hopes of millions, and how he offered a salvation path for oppressed and oppressors alike,” Obama said to an audience of tens of thousands gathered on the mall despite a steady drizzle. “His words belong to the ages, possessing a power and prophecy unmatched in our time.”

As the first black president, Obama’s remarks were particularly poignant, coming after he and first lady Michelle Obama as well as King’s family and followers sounded the cast iron bell from a Birmingham, Ala., church where four black girls were killed in a bombing in September 1963.

Obama spoke after several speeches delivered by several members of the King family, as well as Presidents Clinton and Carter, offering his personal reflections on the progress achieved and the racial divisions and challenges that the country continues to face.

In reverential terms, Obama spokes of the “courage” it took for civil rights heroes like King to peacefully challenge racial segregation and discrimination, mentioning by name Rep. John Lewis, D-Ga.; Medgar Evers, James Chaney, Andrew Goodman and Michael Schwerner.

The nation has come a long way since the days when those freedom marchers fought against discrimination and segregation and police used fire hoses to break up protests and four little girls lost their lives, Obama said.

“Because they kept marching, America changed,” he said. “… Because they marched, city councils changed and state legislatures changed and Congress changed and, yes, eventually the White House changed.

“Those who would say little changed…that dishonors the courage and the sacrifice of those who paid the price to march in those years,” he said. “They did not die in vain. Their victory was great.”

But to suggest that the civil rights work of the nation is done would also dishonor “these heroes,” the president argued. Too many blacks and minorities in America are having trouble entering the middle class, he said, and he blamed Wall Street greed and the country’s economic policies for limiting their opportunities.

Calling increased access to economic opportunity a second goal of original March on Washington, Obama said securing the gains the country has made on racial injustice requires “vigilance, not complacency.”

“For what does it profit a man, Dr. King would ask, to sit at an integrated lunch counter if he can’t afford the meal? Obama said.

Obama encouraged Americans of all nationalities and faiths to “keep on marching.”

In his remarks, Clinton exhorted Americans to “stop complaining and start trying to fix” the country.

He also said King's speech fifty years ago motivated him to seek public service.

“This march, and that speech, changed America,” Clinton said. “They opened minds, they melted hearts and they moved millions – including a 17-year-old boy watching alone in his home in Arkansas.”

Carter, a strong advocate for non-violent forms of protest throughout his life, said King's “I Have a Dream Speech” helped not just black Americans, but “in truth, he helped to free all people.”

Former President George W. Bush, was invited but is still recovering from a heart procedure, said in a statement that “our country has come a long way since that bright afternoon 50 years ago; yet our journey to justice is not complete.”

A number of celebrities and musicians attended the ceremony.

Oprah Winfrey, one of the day's headliners, said King force the nation “to wake up, look at itself and eventually change.”

King recognized that Americans all share the same dreams and hopes for their families regardless of race, she said, and he was right to promote a message that all Americans lives and destinies are intertwined and will rise and fall together.