Aretha Franklin, poet Nikki Giovanni and President Barack Obama were among those who attended the more than four-hour ceremony. King's children and other leaders spoke before the president, invoking his "I Have a Dream" speech and calling upon a new generation to help fully realize that dream.
Some in the crowd arrived as early as 5 a.m., and the crowd eventually overflowed beyond the park gates. Some women wore large Sunday hats for the occasion.
The president arrived late morning with his wife and two daughters, which drew loud cheers from those watching his entrance on large screens.
Cherry Hawkins traveled from Houston with her cousins and arrived at 6 a.m. to be part of the dedication. They postponed earlier plans to attend the August dedication, which was postponed because of Hurricane Irene.
"I wanted to do this for my kids and grandkids," Hawkins said. She expects the memorial will be in their history books someday. "They can say, `Oh, my granny did that."'
Hawkins, her cousin DeAndrea Cooper and Cooper's daughter Brittani Jones, 23, visited the King Memorial on Saturday after joining a march with the Rev. Al Sharpton to urge Congress to pass a jobs bill.
"You see his face in the memorial, and it's kind of an emotional moment," Cooper said. "It's beautiful. They did a wonderful job."
A stage for speakers and thousands of folding chairs were set up on a field near the memorial along with large TV screens. Most of the 10,000 chairs set out appeared to be full. Many other people were standing.
The August ceremony had been expected to draw 250,000, though organizers anticipated about 50,000 for Sunday's event.
Actress Cicely Tyson said her contemporaries are passing the torch to a new generation and passed the microphone to 12-year-old Amandla Stenberg. The girl recalled learning about the civil rights movement in school and named four young girls killed in a 1963 church bombing in Birmingham, Ala.
"As Dr. King said at their funeral, `They didn't live long lives, but they lived meaningful lives,"' Amandla said. "I plan to live a meaningful life, too."
About 1.5 million people are estimated to have visited the 30-foot-tall statue of King and the granite walls where 14 of his quotations are carved in stone. The memorial is the first on the National Mall honoring a black leader.
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."
King's "Dream" speech during the March on Washington galvanized the civil rights movement.
King's older sister, Christine King Farris, said she witnessed a baby become "a great hero to humanity." She said the memorial will ensure her brother's legacy will provide a source of inspiration worldwide for generations.
To young people in the crowd, she said King's message is that "Great dreams can come true and America is the place where you can make it happen."
King's daughter, the Rev. Bernice King, said her family is proud to witness the memorial's dedication. She said it was a long time coming and had been a priority for her mother, Coretta Scott King, who died in 2006.
Bernice King and her brother Martin Luther King III said their father's dream is not yet realized. Martin Luther King III said the nation has "lost its soul" when it tolerates vast economic disparities, teen bullying, and having more people of color in prison than in college.
He said the memorial should serve as a catalyst to renew his father's fight for social and economic justice.
"The problem is the American dream of 50 years ago ... has turned into a nightmare for millions of people" who have lost their jobs and homes, King said.
The nation's first black president, who was just 6 years old when King was assassinated in 1968, saluted King as a man who pushed the nation toward what it ought to be and changed hearts and minds at the same time.
"He had faith in us," Obama said. "And that is why he belongs on this Mall: Because he saw what we might become."
Giovanni read her poem "In the Spirit of Martin," and Franklin sang.
Early in the ceremony, during a rendition of "Lift Every Voice and Sing," the crowd cheered when images on screen showed Obama on the night he won the 2008 presidential election.
Obama, who credits King with paving his way to the White House, left a copy of his inaugural speech in a time capsule at the monument site. He said King was a man who "stirred our conscience" and made the Union "more perfect."
But the Rev. Al Sharpton said the dedication was not about Obama but the ongoing fight for justice. He called for people from around the world to walk through the stone of hope and emerge to see "the face that brought us from the back of the bus to the White House."