SAN FRANCISCO -- An estimated crowd of 5,000 protesters angered by the passage of a state measure banning gay marriage marched through downtown San Francisco during rush hour, carrying signs and snarling traffic, according to San Francisco police.

The group of protesters began gathering at U.N. Plaza near 7th Street and Market Friday afternoon at around 4:30 p.m. for a rally that was scheduled to start an hour later. Just before 6 p.m., the demonstration left the plaza and moved into Market Street.

The protesters were voicing their angst over the passage of Proposition 8, which would amend the California Constitution to define marriage as only between a man and a woman. Prop. 8 overrides a state Supreme Court ruling that briefly gave same-sex couples the right to wed.

The march route goes up Market to Castro Street before turning left down to 18th Street and finally ending in a second rally at Dolores Park.

Walking with a sign that reads "You gave rights to chickens and took away rights from human beings," 42-year-old Lisa Davis of Alameda said she planned on proposing marriage to her same-sex partner during the march.

At around 6 p.m., some stopped and joined hands across Market Street at Ninth Street. The barrier stopped traffic, causing honks from vehicles, some in support and others in an urgency to get past.

Police could not say by that time if anyone had been arrested but officers were keeping an eye on the protest in hopes it would remain peaceful. Officers may briefly close side streets as the demonstrators pass by.

Word of the march spread through blogs and Web sites like Yelp and Facebook urging people to show up.

Those who made it to Dolores Park chanted, played music and danced, prompting the temporary closure of Dolores Street at 19th Street.

Police Lt. Neville Gittens said three people were arrested for being drunk in public but added that they did not appear to be part of the protest, just observing.

Gittens said that by about 8 p.m. the protesters were heading back toward United Nations Plaza. A smaller group broke off and returned to the Castro, setting up a makeshift stage for speakers and blocking the intersection of 18th Street and Castro. Police said they do not plan to make any arrests.