Hundreds of gay and lesbian couples married across New York State on Sunday in a key milestone for advocates hoping to legalise gay marriage nationwide.

Niagara Falls, which shares the most powerful waterfalls in North America with a Canadian city of the same name, hosted the first marriage after the stroke of midnight with the cascades illuminated in gay pride rainbow colors as the backdrop.

"This wasn’t done with just the two of us," Kitty Lambert, one of the newlyweds who married her long-time partner Cheryle Rudd, told The Buffalo News newspaper.

"Every single person here played a part in getting this law passed."

Cities and towns across the Empire State plan to open offices to issue the state's first marriage licenses to gay couples after New York legalized same-sex marriage on June 24 in a nail-biting vote.

It became the sixth US state, and the most populous, to legalise gay marriage.

In New York City alone - the largest city in the United States with eight million residents - 823 couples have registered in advance to get their marriage licenses on Sunday.

It will mark a unique day in the Big Apple's history.

The city's last marriage records were 621 on Valentine's Day (February 14) 2003 and 610 unions on August 8, 2008, because of the popularity of the date 08/08/08.

New York City Mayor Michael Bloomberg will officiate one of the Sunday marriages at Gracie Mansion, the mayor's official residence, between two male colleagues.

In the Bronx, the Reverend Carmen Hernandez, who proudly bills herself as the 'First Lesbian' of the tough neighborhood, is giddy with excitement about her upcoming wedding.

Born on the Spanish-speaking US island of Puerto Rico, Hernandez has lived nearly all of her life in the Bronx and is a well-known local activist in the gay and lesbian community.

"I am the First Lesbian in the Bronx, fighting homophobia in the Latino community, where machismo rules," said Hernandez, a youth pastor at the Methodist Church of Resurrection.

"I've even fought against some gay people for selling out to certain politicians."

In addition to the record number of ceremonies, New York City is preparing for several weeks of festivities around an expected wave of same-sex marriages.

Hotels, restaurants, florists and other businesses are offering special deals and a giant wedding event is planned for the following Sunday in Central Park.

The ceremonies will come just two days after President Barack Obama announced that the 'Don't Ask, Don't Tell' ban on gays serving openly in the US armed forces will end on September 20 as he certified that the military was ready to accept gay troops among its ranks.

The 1993 law required gay troops to keep quiet about their sexual orientation or face expulsion from the forces, and an estimated 14,000 service members have been kicked out of the military under the rule.

Gay marriage is not legal under federal laws.

The so-called Defense of Marriage Act only recognises marriage as the union between a man and a woman.

The White House says Obama favors legislation that would repeal the Defense of Marriage Act.

The law also allows states where gay marriage is not permitted to refuse to recognize a legally-sanctioned gay marriage from another state.