The visit by the president and first lady Michelle Obama to a Washington school was among a raft of speeches, tributes and parades on the 30th anniversary of the U.S. holiday commemorating King.
Bernice King, daughter of the Nobel Peace Prize winner, told a standing room-only crowd at Ebenezer Baptist Church in Atlanta, where her father had preached, that blacks faced the erosion of voting rights, environmental injustice and rising gun deaths.
"What would my father say?" asked King, who was frequently interrupted by deafening applause. "He'd say, 'What is taking you all so long?'"
She added, "Now is the time to be resilient, now is the time to be determined."
Coinciding with the King holiday, African-American director Spike Lee and actress Jada Pinkett Smith said they would boycott the Academy Awards ceremony because black actors had been shut out of Oscar nominations for a second straight year.
In Washington, the Obamas took part in a community service event at Leckie Elementary School, which has a large proportion of students from military families. The event included preparing a garden bed and packing bags with books, the White House said.
In Columbia, South Carolina, Democratic presidential candidates Hillary Clinton, Bernie Sanders and Martin O'Malley addressed a King rally at the state capitol a day after their party debate in Charleston.
Clinton told the crowd that deaths of blacks at the hands of police was a civil rights issue. Sanders, a Vermont senator, said that King at the time of his assassination in 1968 was fighting for U.S. economic equality.
"What would he say about a nation where 29 million people don't have health insurance?" he said.
The statehouse commemoration was the first that did not have the Confederate battle flag flying on the grounds. In San Jose, California, a "Celebration Train" will make a 54-mile trek to San Francisco to commemorate King's historic 1965 march from Selma to Montgomery, Alabama, for civil rights.
Religious leaders in Cleveland plan a rally at the Cuyahoga County prosecutor's office to protest police handling of the death of Tamir Rice, 12. A Cleveland police officer shot him in 2014 and a grand jury has declined to indict the officer.