Organizers said people at more than 300 sites in nearly every state were ringing their bells to commemorate Martin Luther King Jr.'s Aug. 28, 1963 "I Have a Dream" speech.
At the National Cathedral in Washington, the central bell tower played "Lift Every Voice and Sing" from the carillon. The bells rang for about 15 minutes to mark the moment when the speech was delivered.
"I remember 50 years ago: the marching, the throngs of people, the speech, the energy," said Party Mason, 69, of Bethesda, Md., who was one of about two dozen people who gathered at the cathedral. "It was amazing, just amazing."
Commemorations were taking place from a mountain in Georgia carved with the likenesses of Confederate leaders to the far reaches of Alaska, where participants rang cow bells along and bear bells in Juneau.
Many of the commemorations were in sync with the hour when King gave his speech, 3 p.m. EDT, though some churches planned to ring their bells at 3 p.m. local time.
Fifty years ago, as King was wrapping up his speech at the Lincoln Memorial, he quoted from the patriotic song, "My Country 'tis of Thee."
King implored his audience to "let freedom ring" from the hilltops and mountains of every state in the nation, some of which he cited by name in his speech.
"When we allow freedom to ring - when we let it ring from every city and every hamlet, from every state and every city, we will be able to speed up that day when all of God's children, black men and white men, Jews and Gentiles, Protestants and Catholics, will be able to join hands and sing in the words of the old Negro spiritual, 'Free at last, free at last, great God almighty, we are free at last," King said in closing.
One of the places cited in King's speech was Georgia's Stone Mountain, a granite outcropping east of Atlanta. The park is a memorial to the Confederacy, including a 17,000-square-foot sculpture of three Confederate leaders carved into the mountainside itself. The Ku Klux Klan held rallies there during the 20th century. Now it's a favorite hiking destination for families white and black.
On Wednesday, about 30 children and adults hiked up the mountain to join a commemoration. At the summit, participants played a recording of King's speech and sang "We Shall Overcome," a spiritual favored by civil rights marchers. They also rang bells.
One hiker, Gail Scotton Baylor, 58, recalled watching King's speech on a black-and-white TV from her family's home in High Point, N.C. As a child, she remembered watching white children eat ice cream in a parlor while she and other black children were served at a side door. She remembered segregated water fountains and bathrooms. She recalled the dejected look on her father's face when a restaurant refused to let him buy hamburgers for his family because he was black.
"I do remember Dr. Martin Luther King saying, 'Free at last, free at last, thank God Almighty, I'm free at last,' and hearing all these people yelling and screaming. And I knew, I knew this was a very important day as a little girl," she said. "And I felt like something good was happening - that something good was going to happen for us as a people, black people. Because even as a child, I knew that something was wrong."
David Soleil, a founder of the Sudbury School of Atlanta, brought students and parents to the march. He read passages from King's speech as they progressed.
"This is a place that has just had a history of racism and pain," he said. "But we're here. And we're fulfilling that dream that freedom is going to ring from Stone Mountain. Even in places that birth hate, that we can still come and birth love."
In Western Pennsylvania, students and professors at Allegheny College celebrated their area's connection to the speech, which mentions "the mighty Alleghenies of Pennsylvania."
Some at the school have wondered whether the line in the speech was a way of acknowledging the school, since several Allegheny students were also active in King's civil rights movement in the early 1960s. The school is located in Meadville, a small town about 90 miles north of Pittsburgh.
Charles B. Ketcham, 87, a former professor of religion at Allegheny who corresponded with King in the early 1960s, said that might be a stretch.
"It was probably a beautiful way to get some alliteration. He was wonderful at that," Ketcham said of King.
Ketcham didn't go to Washington 50 years ago, but followed the speech, and those today honoring it.
"I think we have made tremendous progress. But I do think we have a long way to go," Ketcham said, reflecting back on 1963.
While Alaska wasn't mentioned in King's speech, residents there found a unique way to commemorate the speech.
A small group gathered in the courthouse plaza in Juneau, Alaska, across from the state Capitol, to mark the occasion by ringing bear bells, cowbells and hand bells at the appointed time. Bear bells are used by hikers to make noise to let bears know people are around.
The group - which included the local police chief - then joined hands to sing "We Shall Overcome."
Charmaine Weeks, a housewife, called the 50th anniversary of the speech a momentous occasion. "And I think across the country everyone recognizes that. And it's important that everyone, whether it's in the Last Frontier, you know, or in Washington, D.C., that you mark it, however that is."
At the National Cathedral, some of the women, both black and white, hummed along to "We Shall Overcome," and "Amazing Grace." King preached his final Sunday sermon at the National Cathedral in 1968 before traveling on to Memphis, Tenn., where he was assassinated.
Carin Ruff, 48, of Washington, said a tear came to her eye as she heard the cathedral bells and listened to a radio broadcast of Obama and others speaking on the National Mall.
"It felt like this was the neighborhood place to come for big events," she said. "I think it was a fabulous idea."
Contributing to this report were Associated Press writers Ray Henry in Stone Mountain, Ga., Becky Bohrer in Juneau, Alaska and Kevin Begos in Pittsburgh.