Thousands of people gathered on the National Mall here on the weekend to attend the dedication of the monument to Dr. Martin Luther King Jr., at which President Barack Obama recalled that the life of the activist "tells us that change can come if you don't give up."

Without the "glorious words" of King's 1963 "I Have a Dream" speech on the Mall, "we might not have had the courage to come as far as we have," the president emphasized at the Sunday ceremony.

After more than 15 years of preparation and a month behind schedule due to the bad weather brought by Hurricane Irene, on Sunday singers, poets, civil rights activists and others paid tribute to the Baptist minister who became an iconic civil rights leader.

"We are right to savor that slow but certain progress - progress that's expressed itself in a million ways, large and small, across this nation every single day, as people of all colors and creeds live together, and work together, and fight alongside one another, and learn together, and build together, and love one another," said Obama.

The president, who - with wife Michelle and daughters Malia and Sasha - toured the monument while the crowd attended the celebration on the Mall, insisted that in the face of the current economic situation people should not become "discouraged" and added that King had to confront many more difficulties to achieve his dream.

"And just as we draw strength from Dr. King's struggles, so must we draw inspiration from his constant insistence on the oneness of man; the belief, in his words, that 'we are caught in an inescapable network of mutuality, tied in a single garment of destiny,'" Obama said.

Obama, the first black U.S. president, has a bust of King (1929-1968) in the Oval Office and quotes him frequently in his speeches.

He had paid tribute to King in the days after he took office, in January 2009, when he instituted social volunteer days in memory of the assassinated leader, whose relatives were also at the ceremony on Sunday, a gathering that was enlivened by the singing of Aretha Franklin.

The monument to King is an enormous shaped and carved white stone 9 meters (29 feet) high out of which emerges the figure of the activist.

"And so with our eyes on the horizon and our faith squarely placed in one another, let us keep striving; let us keep struggling; let us keep climbing toward that promised land of a nation and a world that is more fair, and more just, and more equal for every single child of God," Obama said.