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.
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