NEW YORK — Wedding ceremonies for hundreds of gay and lesbian couples were being held across New York State Sunday, with advocates hailing the day as a milestone toward the eventual legalization of gay marriage nationwide.

Sunday marked the first day when a law allowing same sex marriage goes into effect across New York state, including the metropolis of New York City and its environs.

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 famous 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 legalize gay marriage.

"Today the doors of marriage equality swing open in New York. It's no surprise a flood of same-sex couples is expected to flow through," noted the National Gay and Lesbian Task Force in a statement.

"Until you have walked in the shoes of someone who has been denied this fundamental freedom -- a rite that bonds us a people -- it is hard to comprehend just how profoundly moving this moment is. Today is a day to rejoice and celebrate."

In Manhattan, Nancy Mertzel, a 48-year-old lawyer, and partner Yolanda Potasinski, 55, were in line before sunrise awaiting their turn to get married

"We are not anymore second-class citizens in the State of New York. There's still work to do in the rest of the country," Mertzel noted.

Opposite the registry office, a small crowd of about a dozen people booed the happy couples. An orthodox Jewish man held a sign that simply read: "Gay Marriage: Bad Idea."

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, who had pushed for the law, was to officiate one of the Sunday marriages at Gracie Mansion, the mayor's official residence, between two male colleagues.

"Over the years, we have rectified injustices or improved on equality," the mayor told ABC's "This Week" program of the history of US tolerance.

Bloomberg said he thought gay marriage would take off across the United States "simply because of the economics and the young people" who will gain mroe influence on politics in years to come.

"Nothing is ever 100 percent, 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," he said.

Gay marriage is not legal under federal laws. The so-called Defense of Marriage Act only recognizes 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.

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.