NEW YORK (Reuters) - Gay and lesbian couples across New York are gearing up to become bride and bride or groom and groom starting at a minute after midnight on Sunday, when a law making New York the sixth U.S. state to allow such marriages comes into effect.

The expected rush of weddings across New York state has sent marriage bureaus scrambling for staff to officiate and judges to approve the marriages.

New York City plans to open its marriage bureaus in all five boroughs on Sunday. Expecting lines around the block, city officials initially set a lottery capping the number of marriages to 764 couples, but later decided to accept all 823 applicants, gay and straight alike.

In preparation for the crush of media and spectators, police on Friday installed barricades near the Manhattan marriage bureau, which is set to wed 459 couples on Sunday.

Asked if the lines would discourage people, New York Mayor Michael Bloomberg said not to worry, despite the sweltering heat wave engulfing the city and much of the region.

"It's going to be hot and it's going to be a lot of fun, and you joke about the lines," Bloomberg said on a radio show.

Bloomberg, who had pushed for the same-sex marriage law, is set to marry two men who belong to his City Hall inner circle at a ceremony at Gracie Mansion, the mayor's official residence, on Sunday evening.

"It'll be a nice ceremony ... there will be some drinks," Bloomberg said.

Kitty Lambert and Cheryl Rudd, from Buffalo, New York, are set to be married as soon as the clock strikes 12:01 on Sunday morning at the base of Niagara Falls, used by generations of Americans as a honeymoon destination.

They said their marriage represents a major step for gay rights in the United States.

"This is an amazing moment," Lambert said. "We're achieving that real American Dream to be treated like everybody else and be protected under all those laws."

She and Rudd, who share five grown children from previous marriages before coming out as gay, will mark the occasion by lighting the world-renowned water cascade with rainbow-colored lights, the colors of which are a symbol of gay pride.

On Monday, 42 other gay couples plan to say "I do" together during a joint ceremony at Niagara Falls.

Civil liberties activists in New York say the state's legalization of same-sex marriage sends a message to the U.S. Congress that it must repeal a federal law defining marriage as between a man and a woman.

The Defense of Marriage Act was signed into law in 1996 by then-Democratic President Bill Clinton. U.S. President Barack Obama has said he would support a bill meant to repeal the law.

The law 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.

(Additional reporting by Neale Gulley in Buffalo,; editing by Mark Egan and Mohammad Zargham)

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