By Daniel Lovering

PORTLAND, Maine (Reuters) - Gay and lesbian couples eager to become the first to wed under Maine's newly enacted same-sex marriage law gathered Friday night outside City Hall in Portland, the state's largest city, waiting until the stroke of midnight to exchange their vows.

About a half dozen couples huddled with friends and family in freezing temperatures outside the building before the doors to the city clerk's office were opened at 10 p.m. local time, and by 11:30 p.m. the number of prospective brides and grooms had more than doubled and was expected to grow.

"We've been together for 30 years, and never thought that this country would allow marriages between gay couples," said Roberta Batt, 71, an antiques dealer and retired physician with silver hair and round eyeglasses. She planned to marry her longtime partner, Mary, who stood nearby.

"We're just very thankful to the people of Maine, and I hope the rest of the country goes the way this state has," she added.

Along with voters in Maryland and Washington state, Maine residents approved ballot initiatives legalizing same-sex unions on November 6, Election Day, making them the first states to extend marriage rights to gay couples by popular vote.

Nine of the 50 U.S. states plus the District of Columbia, have now legalized gay marriage. Another 31 states have passed constitutional amendments banning it.

The first pair in line to tie the knot in civil ceremonies to be held in Portland after midnight, Steven Bridges, 42, and Michael Snell, 53, were greeted by a jazz trio playing in the lobby.

Reaching a large table spread with wedding corsages offered to arriving couples, they pinned purple flowers on each other's matching black T-shirts, emblazoned with colorful designs and the words, "Love is Love."

Bridges said their impending wedding meant "that we're equal, that we're the same. We're no better, no worse, that we're finally recognized by the state of Maine as a couple."

City clerks' offices around Maine scheduled extra office hours to accommodate same-sex couples rushing to wed.

In Bangor, the city clerk's office was planning to be open on Saturday from 6 a.m. to 8 a.m. for residents to obtain marriage licenses. The Brunswick town clerk's office was set to be open from 9 a.m. to noon on Saturday by appointment. As of midday on Friday, five same-sex couples had booked appointments, the office said.

More lavish same-sex weddings are being booked starting in the spring at the On the Marsh Bistro in Kennebunk, said owner Denise Rubin.

"We support it wholeheartedly," she said. "We look forward to being part of a whole new wave of wonderful thinking."

The tide of public opinion has been shifting in favor of allowing same-sex marriage. In May, President Barack Obama became the first U.S. president to say he believed same-sex couples should be allowed to get married.

A Pew Research Center survey from October found 49 percent of Americans favored allowing gay marriage, with 40 percent opposed. The U.S. Supreme Court has agreed to review two challenges to federal and state laws that define marriage as a union between a man and a woman.

The nation's highest court said this month it will review a case against a federal law that denies married same-sex couples the federal benefits that heterosexual couples receive. It also will look at a challenge to California's ban on gay marriage, known as Proposition 8, which voters narrowly approved in 2008.

Washington state's law legalizing same-sex unions took effect on Sunday, December 9, and Maryland's law takes effect on January 1, 2013.

(Reporting and writing by Ellen Wulfhorst; editing by Paul Thomasch, David Gregorio and Todd Eastham)