Chief among them was the 45th annual service for the civil rights leader at the Atlanta church where King was pastor.
There, Bernice King stressed her father's commitment to nonviolence, saying that after the 1956 bombing of the family's home in Montgomery, Ala., her father stood on the porch and urged an angry, armed crowd to fight with Christian love — not guns.
"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 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 Church.
He urged those listening 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."
The Atlanta service 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!" Afterward many stayed to watch Obama's second inauguration on a big-screen TV.
In Columbia, S.C., civil rights leaders paused during their annual King Day rally to watch the inauguration on a big screen.
"You feel like anything is possible," Jelin Cunningham, a 15-year-old girl, said of Obama's presidency.
Elsewhere, visitors thronged the National Civil Rights Museum in Memphis, Tenn., the city where King was assassinated in 1968. In Detroit, students beautified schools. Others painted murals honoring King in Arkansas, donated food bank items in Texas, and conducted a community health fair in Pennsylvania.
Associated Press writers Jeffrey Collins in Columbia, S.C.; Jessica Gresko in Washington; and Adrian Sainz in Memphis, Tenn., contributed to this report.