The president, who credits King with paving his way to the White House, left a copy of his inaugural speech in a time capsule at the monument earlier in the day.
Many who crowded in to see the president and hear Aretha Franklin and Stevie Wonder sing chanted "four more years" when Obama arrived. And they said the talk of economic fairness resonated with their own lives.
Joyce Lansdown, 61, a retired federal worker from Chantilly, Va., brought her daughter and granddaughter to the ceremony. She was glad Obama and others mentioned the importance of caring for seniors during the economic downturn.
"My heart goes out to them," she said. "My momma is still living on her little Social Security check."
Patricia Johnson, 50, drove with her godfather's granddaughter from Twinsburg, Ohio, to see the president and King's family.
"It seems that President Obama as a young president is following King's footsteps," she said. "I think we can continue to learn a lot from (King's) example."
Her young family friend, 13-year-old Faron Bouldin, wiped tears from her eyes as a recording of King's full "Dream" speech played on large TV screens after Obama spoke.
"It feels really important for me," Bouldin said of King's message.
Some 10,000 chairs set up in a field near the memorial site were all filled. Many others stood in overflow sections.
The August ceremony when the memorial opened had been expected to draw 250,000, though organizers anticipated about 50,000 for Sunday's event.
Violinist Miri Ben-Ari performed an original composition written for the event and the song "Bus Passed" with spoken word artists Poem-Cees. Poet Nikki Giovanni read her poem "In the Spirit of Martin."
Wonder, Sheryl Crow and James Taylor performed in a concert after the dedication.
The Rev. Al Sharpton called for people from around the world to see the monument's "stone of hope."
"When you walk through, you see a man standing in a posture of faith," he said. "Faith that brought us from the back of the bus to the White House."
Before the dedication, the King siblings walked through the memorial plaza with Obama, first lady Michelle Obama, and their two daughters.
Bernice King said her family was proud to witness the memorial's dedication and hope it will spur action to solve the nation's problems.
Echoing her father's words, she told the crowd, "One day we'll all be able to say 'Free at last, free at last, thank God almighty, we are all free at last."
Brett Zongker can be reached at http://www.twitter.com/DCArtBeat
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