Cheryle Rudd, left, and Kitty Lambert, right, react after being married at the stroke of midnight Sunday at the brink of Niagara Falls, in Niagara Falls, N.Y., by the city's mayor, Paul Dyster. Doug Benz/Reuters

Hundreds of gay couples dressed in morning suits, gowns and T-shirts recited vows in emotion-choked voices and triumphantly hoisted their long-awaited marriage certificates on Sunday as New York became the sixth and largest state to recognize same-sex weddings.

Couples began saying "I do" at midnight from Niagara Falls to Long Island, though New York City became the sometimes raucous centre of action by daybreak Sunday as couples waited on a sweltering day for the chance to exchange vows at the city clerk's office.

Thousands of protesters rallied in several cities around the state, a signal that the long fight for recognition may not be over just yet.

But a party atmosphere reigned in the lobby of the Manhattan clerk's office, with cheers and applause breaking out whenever a couple was handed their white-and-blue wedding certificate.

Balloons floated overhead. One couple wore matching kilts; another wore sparkly crowns. Children scurried up and down the lobby; workers with bullhorns called out the numbers of each couple.

Poignant signs of pent-up emotion were common from couples who had in some cases waited for years to wed. Couples cried and voices quavered. Newlywed Douglas Robinson exclaimed, "You bet your life I do!" when asked if he would take Michael Elsasser as his spouse.

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

Kopelov arrived in a wheelchair and stood with the assistance of a walker. During the service, Siegel wrapped her hand in Kopelov's and they both grasped the walker.

Witnesses cheered and wiped away tears after the two women vowed to honour and cherish each other as spouses and then kissed.

"I am breathless. I almost couldn't breathe," Siegel said after the ceremony. "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."

Outside afterward, Siegel raised her arms exultantly as Kopelov, in the wheelchair, held out a marriage certificate.

New York voted last month to legalize gay marriage

New York's adoption of legal same-sex marriage is viewed as a pivotal moment in the national gay rights movement and was expected to galvanize supporters and opponents alike. The state joined Connecticut, Iowa, Massachusetts, New Hampshire and Vermont, along with Washington, D.C., when it voted last month to legalize gay marriage.

Protest rallies were planned in Manhattan, Buffalo, Rochester and Albany on Sunday afternoon. Gay marriage opponents unhappy that Gov. Andrew Cuomo and lawmakers legalized same-sex marriage last month are calling for a statewide referendum on the issue.

Several hundred people crowded into the street across from Cuomo's Manhattan office to protest the new law. They waved signs saying "Excommunicate Cuomo" and chanted "Let the people vote!"

Hundreds more protested on the steps of Buffalo's City Hall and at the state capitol in Albany.