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Hundreds of gay couples have wed in New York after it became the sixth and most populous state in the US to recognise same-sex unions as legal.

Newly-weds brandished their marriage certificates as ceremonies were held from Manhattan to Niagara Falls.

Civil rights activists argue the US Congress should now repeal a federal law defining marriage as only existing between a man and a woman.

But thousands of people condemned the weddings at rallies in the state.

Just after midnight, Kitty Lambert, 54, and Cheryle Rudd, 53, from Buffalo, New York, were married at Luna Island, making it the state's first same-sex union.

Luna Island, at the foot of Niagara Falls, is a traditional wedding destination.

"We're achieving that real American dream to be treated like everybody else and be protected under all those laws," Ms Lambert said after a ceremony held to the beat of pop star Lady Gaga's The Edge of Glory.

In Manhattan, the first couple to marry were Phyllis Siegel, 77, and Connie Kopelov, 85, who have been together for 23 years.

Ms Kopelov arrived in a wheelchair and stood with the assistance of a walker.

"I am breathless, I almost couldn't breathe," Ms Siegel said later.

"It's mind-boggling. The fact that's it's happening to us - that we are finally legal and can do this like everyone else."

New York City Mayor Michael Bloomberg, who had pushed for the gay marriage law to be adopted, was officiating at the marriage of two male colleagues in his official mayoral residence.

He told ABC television he thought gay marriage would take off across the US "simply because of the economics and the young people" who would gain more influence on politics in years to come.

"Nothing is ever 100% but this is a trend that's going, and it's going to grow very rapidly, partially because New York is such a bellwether and so visible."

New York recognised same-sex weddings in a close state Senate vote on 24 June after strong lobbying by Governor Andrew Cuomo and activists.

The National Organization for Marriage held rallies in New York City, Albany, Rochester and Buffalo on Sunday, accusing Mr Cuomo and lawmakers of redefining marriage without consulting voters.

Protesters chanted "Let the People Vote!" at the rallies.

A gathering in New York City that started with several hundred people opposite Mr Cuomo's Manhattan office quickly swelled to thousands.

They waved signs saying "Excommunicate Cuomo" and "God cannot be mocked".

"I'm worried that the younger generations will think this is normal, and it's not," said Gloria Sanchez, 35, of Brooklyn. "It's wrong. It's a sin."

Outside the capitol in Albany, about 400 people gathered in a park under a banner reading "Marriage(equals)Man and Woman", topped with a "Don't Tread on Me" flag.

Connecticut, Iowa, Massachusetts, New Hampshire, Vermont and the District of Columbia all legalised gay weddings earlier, while Delaware, Hawaii, Illinois and New Jersey have approved gay civil unions.

But gay marriage is specifically banned in 39 states.

The federal Defense of Marriage Act signed into law in 1996 by former President Bill Clinton prohibits same-sex couples from receiving marriage-based federal benefits such as social security survivor benefits, health benefits and the right to file taxes jointly.