The crowd was part of a nationwide protest against the Proposition 8 referendum, which defined marriage as between a man and a woman.
Protesters in Washington gathered Saturday afternoon outside the U.S. Capitol before marching through the city carrying signs reading "Allies against Prop H8" and chanting "One, two, three, four, love is what we're fighting for!"
Kellan Baker, an organizer who addressed the crowd at Lafayette Park, said "no one in this country should be a second-class citizen."
Besides protesting the California vote, Baker said the demonstrations are meant to urge President-elect Barack Obama and other officials to repeal the 1996 federal Defense of Marriage Act and support gay rights.
Hundreds of people gathered outside Baltimore's city hall to protest the California ban.
Mike Bernard, of Baltimore, who married his partner in Canada this year, was one of several people who shared their personal stories with the crowd. In the long run, Proposition 8 may be a good thing for those fighting for gay marriage in the United States, he said. Many thought a liberal state like California would never ban gay marriage, he said, but now they may be shocked into action.