In Los Angeles, protesters clustered shoulder to shoulder near City Hall before setting off on a downtown march, chanting and carrying rainbow flags and signs bearing messages such as "No More Mr. Nice Gay" and "No on Hate."
"It's invigorating and exciting to see us unite as a people," Christine Pease, 39, said as she handed out stickers with a yellow equal sign to demonstrators outside City Hall. "I hope that it shows there are a lot more people affected by the choices we make on a ballot."
The Los Angeles Police Department estimated that 10,000 to 12,000 people attended the event, well below the 40,000 the department had expected.
Still, demonstrators called the event a success, noting that participants had been galvanized by a loosely organized grass-roots campaign that sprang up after the Nov. 4 election.
"Considering it started on Facebook and became as organized as it was, it's pretty amazing," said Dave Coleman, 43.
A representative of the Proposition 8 campaign said the protests would have little effect. "They can protest all they like, and it doesn't change the fact that Prop. 8 has passed and the election is now over," said Frank Schubert, manager for the Yes on Proposition 8 campaign.
In San Francisco, a crowd estimated by police at 7,500 converged on the city's civic center, some wearing T-shirts emblazoned with "Milk," a reference to the county's first openly gay supervisor, Harvey Milk, who was assassinated 30 years ago.
Although a march had not been planned, about noon a large portion of the crowd set off along Market Street and split into two, one headed for the Castro District and another for the Embarcadero. Police made several arrests for holding up traffic, but the event was otherwise peaceful.
Demonstrators also gathered in Boston, New York and other cities across the nation, the Associated Press reported.
Across California, the rallies took on a carnival-like atmosphere. About 200 protesters gathered at Costa Mesa's South Coast Plaza. In Sacramento, police estimated that 1,500 marched peacefully on the Capitol.
Amid the throngs, thousands of little scenes played out. A sampling:
Saturday's date held a special significance for Los Angeles marchers Carol Kirkman and Margaret Gonzalez. It was supposed to be their wedding day.
Kirkman and Gonzalez, both 48 from Lawndale, began planning their wedding two months ago and didn't worry about Proposition 8. "We didn't even think twice that this would pass," Kirkman said.
The plans called for an intimate ceremony -- about 10 guests -- and the couple decided to wear matching suits, Kirkman in black, Gonzalez in white. Instead, they joined the thousands marching through downtown wearing sneakers, khaki shorts and T-shirts.
The rallies brought out a diverse crowd that included straight as well as gay families.
Pam Chan and her husband brought their two children -- Kalea, 6, and Koa, 2 -- to the demonstration in San Francisco.