The 30-foot, 1,600-ton granite monument opened in August on the National Mall, but the opening ceremony was postponed because of Hurricane Irene.
President Barack Obama, relatives of King, Aretha Franklin and poet Nikki Giovanni were among those honoring the legacy of the nation's foremost civil rights leader.
Some attendees started lining up at 5 a.m. and even earlier Sunday morning.
The memorial - the first on the National Mall honoring a black leader - has 14 of King's quotations carved in stone.
The sculpture of King with his arms crossed appears to emerge from a stone extracted from a mountain. It was carved by Chinese artist Lei Yixin. The design was inspired by a line from the famous "I Have a Dream" speech in 1963: "Out of the mountain of despair, a stone of hope."
The Associated Press contributed to this report.