Massachusetts has become the first and only US state to allow same-sex marriages.
At the stroke of midnight, cheers erupted among hundreds of gay and lesbian couples waiting to apply first thing in the morning for marriage licenses at City Hall in the famously liberal community of Cambridge, Massachusetts.
It is expected the first legal gay weddings will take place after courthouses open across the state.
Passing cars honked their horns and a crowd of onlookers choked the sidewalks outside the 19th-century building. Across the street, about a half-dozen anti-gay demonstrators held signs like "God Hates Fags."
"If they're going to allow this, then why not incest? Why not have people marry animals? Why not polygamy?" asked one protester.
Thousands of same-sex couples were married at San Francisco City Hall earlier this year, but the marriages were not recognisd by the state of California.
A mayor in New York state is being prosecuted after performing gay marriages in February.