Crowds gathered near public buildings in cities large and small, including Boston, San Francisco, Chicago and Fargo, to vent their frustrations, celebrate gay relationships and renew calls for change.
"Civil marriages are a civil right, and we're going to keep fighting until we get the rights we deserve as American citizens," said Karen Amico, one of several hundred protesters in Philadelphia, holding up a sign reading "Don't Spread H8".
"We are the American family, we live next door to you, we teach your children, we take care of your elderly," said Heather Baker a special education teacher from Boston who addressed the crowd at Boston's City Hall Plaza. "We need equal rights across the country."
Connecticut, which began same-sex weddings this past week, and Massachusetts are the only two states that allow gay marriage. The other 48 states do not, and 30 of them have taken the extra step of approving constitutional amendments. A few states allow civil unions or domestic partnerships that grant some rights of marriage.
Protests following the vote on Proposition 8 in California, which defined marriage as between a man and a woman, have sometimes been angry and even violent, and demonstrators have targeted faiths that supported the ban, including the Mormon Church.
However, representatives of Join the Impact, which organized Saturday's demonstrations, asked supporters to be respectful and refrain from attacking other groups during the rallies.
Seattle blogger Amy Balliett, who started the planning for the protests when she set up a Web page three days after the California vote, said persuasion is impossible without civility.
"If we can move anybody past anger and have a respectful conversation, then you can plant the seed of change," she said.
Balliett said supporters in 300 cities in the U.S. and other countries were holding marches, and she estimated 1 million people would participate, based on responses at the Web sites her group set up.
"We need to show the world when one thing happens to one of us, it happens to all of us," she said.
The protests were widely reported to be peaceful, and the mood in Boston was generally upbeat, with attendees dancing to the song "Respect." Signs cast the fight for gay marriage as the new civil rights movement, including one that read "Gay is the new black."
But anger over the ban and its backers was evident at the protests.
One sign in Chicago, where several thousand people gathered, read: "Catholic Fascists Stay Out of Politics."
"I just found out that my state doesn't really think I'm a person," said Rose Aplustill, 21, a Boston University student from Los Osos, Calif., who was one of thousands at the Boston rally.
Chris Norberg, who married his partner in June, also referred to the racial divisions that arose after exit polls found that majorities of blacks and Hispanics supported the constitutional ban on same-sex marriage.
"They voted against us," Norberg said.
In Salt Lake City, where demonstrators gathered just blocks from the headquarters of the Mormon Church, one sign pictured the city's temple with a line adapted from former Republican vice president candidate Sarah Palin: "I can see discrimination from my house."
More than 500 demonstrators in Washington marched from the U.S. Capitol through the city carrying signs and chanting "One, two, three, four, love is what we're fighting for!"
A public plaza at the foot of New York's Brooklyn Bridge was packed by a cheering crowd of thousands, including people who waved rainbow flags and wore pink buttons that said "I do."
Protests were low-key in North Dakota, where people lined a bridge in Fargo carrying signs and flags.
Mike Bernard, who was in the crowd of hundreds at City Hall in Baltimore, said Proposition 8 could end up being a good thing for gay rights advocates.
"It was a swift kick in the rear end," he said.
In Los Angeles, protesters gathered near City Hall before marching through downtown. Police said 10,000 to 12,000 people demonstrated.
Supporters of traditional marriage said the rallies may have generated publicity but ultimately made no difference.
"They had everything in the world going for them this year, and they couldn't win," said Frank Schubert, co-manager of the Yes on 8 campaign in California. "I don't think they're going to be any more successful in 2010 or 2012."
In Chicago, Keith Smith, 42, a postal worker, and his partner, Terry Romo, 34, a Wal-Mart store manager, had photos of a commitment ceremony they held, though gay marriage is not legal in Illinois.
"We're not going to wait for no law," Smith said. "But time's going to be on our side and it's going to change."
By Associated Press Writer Jay Lindsay; contributing to this report were AP writers Rupa Shenoy in Chicago, Adam Goldman in New York, JoAnn Loviglio in Philadelphia, Sarah Brumfield in Baltimore, Linda Ashton in Salt Lake City, Blake Nicholson in Bismarck, N.D., Tom Verdin in Sacramento, Calif., and Kamala Lane in Washington