"This apostle of nonviolence perhaps introduced one of the bravest experiences of gun control that we've ever heard of in the history of our nation," she said.
The service also kicked off a year of celebrations of the 50th anniversary of King's "I Have a Dream" speech, delivered Aug. 28, 1963, in Washington. Students led by King's great-niece Farris Christine Watkins delivered sections of the speech in turn.
By the end, the crowd was on its feet, shouting, "Free at last! Free at last! Thank God Almighty, we are free at last!"
The keynote speaker was the Rev. Samuel Rodriguez, president of the National Hispanic Christian Leadership Conference, a socially conservative evangelical association. It marked the first time a Latino had been invited to deliver the King Day address at Ebenezer Baptist.
He urged the audience to complete King's dream.
"Silence is not an option when 30 million of our brothers and sisters live in poverty," he said. "Silence is not an option when 11 million undocumented individuals continue to live in the shadows."
Around the country, parades, service projects and memorials marked the holiday.
Visitors from as far as Europe thronged the National Civil Rights Museum in Memphis. In Detroit, students beautified schools. Others painted murals honoring King in Arkansas, donated items to a food bank in Texas, and conducted a community health fair in Pennsylvania.
More than 500 people rallied outside the Alabama Capitol in Montgomery, where state employee Jessie Harris declared Obama's presidency was a sign of progress in "living the dream" that King spoke about.
"We have come far, but the struggle is not over," Harris said.
Associated Press writers Jeffrey Collins in Columbia, S.C.; Jessica Gresko in Washington; and Adrian Sainz in Memphis, Tenn., contributed to this report.