President Barack Obama hailed Martin Luther King Jr Wednesday for saving America from oppression but said "constant vigilance" was needed to keep the civil rights icon's dream of equality alive.
Fifty years after the "I have a dream speech," America's first black president stood on the steps of the Lincoln Memorial in Washington, where King made an appearance in 1963 which changed American history.
"He offered a salvation path for oppressed and oppressors alike. His words belong to the ages, possessing a power and prophecy unmatched in our time," Obama said.
Obama also remembered the thousands of African Americans who joined King's March on Washington to demand their rights and to wake their country's "long slumbering conscience."
The president, who has faced some criticism for not doing more to help the African American community, which remains plagued by poverty and barriers to advancement, dismissed arguments that little had changed for blacks since King spoke.
"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," he said.
But he also argued that much work remained to be done for King's dream to be fulfilled.
"We would dishonor those heroes as well to suggest that the work of this nation is somehow complete.
"The arc of the moral universe may bend towards justice, but it doesn't bend on its own.
"To secure the gains this country has made requires constant vigilance, not complacency," Obama said.
The president was joined at the ceremony on the National Mall in the center of Washington by former presidents Jimmy Carter and Bill Clinton.
Carter bemoaned the "racist bullet" that claimed King's life in 1968.
Clinton said that it was time to open the "stubborn gates" barring wider opportunity.
"The choice remains as it was on that distant summer day 50 years ago. Cooperate and thrive or fight with each other and fall behind."
© AFP 2013