Denver, Colorado - John Lewis, a black congressman who shared the podium when Martin Luther King Jr delivered his famous I Have A Dream speech, called Thursday for Democrats to "march on the ballot box" to elect the country's first African-American president.

Lewis spoke at Denver's Denver's Invesco Field on the 45th anniversary of the event, just hours before Senator Barack Obama, 47, was to accept the Democratic Party's presidential nomination.

Obama is the first black presidential nominee of a major party in the United States.

"With the nomination of Senator Barack Obama ... we are making a down payment on the fulfillment of that dream," Lewis told the crowd assembled on the football field in the late afternoon sun.

By the time of Obama's speech at 8 pm (0200 GMT Friday), the crowd was expected to swell to 75,000.

King's daughter, the Reverend Bernice King, said that the audience would be witness to what has become of her father's dream: "the acceptance of a Democratic presidential nominee decided not by the color of his skin but by the content of his character."

Obama's nomination marks a changing of the guard from ageing civil-rights leaders who faced death and jail to see black Americans treated equally in US society and institutions.

As segregation laws were repealed and equal opportunities opened up, younger African-Americans have broken down one barrier after another in the recent era.

Lewis noted that he is the last surviving speaker from the 1963 event, which drew 250,000 people to Washington and etched the civil rights movement into the nation's conscience.

"For those of us who stood on the steps of the Lincoln Memorial, or who in the years that followed may have lost hope, this moment is a testament to the power of the vision of Martin Luther King Jr," Lewis said. "It is a testament to the promise of America."

As a backdrop to Obama's speech, the stage is set with Greek columns that resemble the Lincoln Memorial on Washington's mall. But French doors make the podium background look like a famous portico at the White House.

King's son, Martin Luther King III, warned that realizing his father's dream was "not Barack Obama's job alone. American needs more than a great president to realize my father's dream. What America needs is a great America." dpa