Like fans anxious for concert tickets, same-sex couples waited in line for hours Sunday outside Cambridge's City Hall for an event they once thought they'd never get to experience: marriage.

Marcia Hams, 56, and her partner, Susan Shepherd, 52, of Cambridge, showed up at midnight Saturday — a full 24 hours ahead of time — to stake out the first spot in line where the city clerk was to hand out the nation's first state-sanctioned gay marriage (search) applications.

"People do this for Red Sox tickets, concert tickets," said Hams, a health care advocate who has been with Shepherd, a graduate student, for 27 years. "Certainly we can do it for this."

By late Sunday night, about 1,500 people had gathered outside Cambridge City Hall, cheering and clapping as city workers opened its doors to let about 75 couples inside ahead of the midnight deadline. Many in the crowd were family and friends who wanted to join in the festive atmosphere. There were also scores of reporters, and a few protesters stood across the street.

Cambridge (search), across the Charles River from Boston and home to Harvard University, decided to seize the earliest possible moment to begin the process of granting same-sex couples the historic right at the center of legal battles nationwide. Mayor Michael Sullivan (search) planned to help cut a three-tiered wedding cake to mark the occasion, and people around the state also held celebrations.

Outside city hall, Alex Fennell, 27, and her partner, Sasha Hartman, 29, said they were happy they didn't have to wait until later Monday morning to begin the marriage process. Same-sex weddings were expected to begin later Monday morning, once judges grant couples' requests to waive standard waiting periods.

"We came here because I've been waiting seven years and I don't want to wait another day, another second," said Fennell, a lawyer from Boston. "For me, it's excitement and gratitude. It's nothing I ever thought we would be able to do."

Massachusetts was thrust into the center of the nationwide debate on gay marriage when the state's Supreme Judicial Court issued its 4-3 ruling in November that gays and lesbians had a right under the state constitution to wed.

In the days leading up to Monday's deadline for same-sex weddings to begin, opponents looked to the federal courts for help in overturning the ruling. On Friday, the U.S. Supreme Court declined to intervene.

The Massachusetts ruling touched off a frenzy of gay marriages across the country earlier this year, emboldening officials in San Francisco, upstate New York and Portland, Ore., to issue marriage licenses as acts of civil disobedience. Even though courts ordered a halt to the wedding march, opponents pushed for a federal constitutional ban on gay marriage, which President Bush (search) has endorsed.

The ruling also galvanized opponents of gay marriage in Massachusetts, prompting lawmakers in this heavily Democratic, Roman Catholic state to adopt a state constitutional amendment that would ban same-sex marriage but legalize Vermont-style civil unions. The earliest it could wind up on the ballot is 2006 — possibly casting a shadow on the legality of thousands of gay marriages that could take place in the intervening years.

As of Monday, Massachusetts joins the Netherlands, Belgium and Canada's three most populous provinces as the only places worldwide where gays can marry, though the rest of Canada is expected to follow soon.

Across the state on Sunday, gay-rights advocates held "Countdown to Equality" parties to celebrate the impending nuptials and to keep attention focused on the political fights ahead.

"I have a younger crowd of friends and I wanted to create some awareness," said Josiah Richards, who was hosting a barbecue for about 35 people in Boston's West Roxbury neighborhood on Sunday.

Several churches held ceremonies honoring gay parishioners and recognizing the fight they've waged for marriage rights.

Robert Compton and David Wilson — one of the seven plaintiff couples in the lawsuit that led to the state court's landmark ruling — attended services at Arlington Street Church in Boston a day before they will exchange vows in the church.

Monday marks the culmination of a legal battle by the couples that began in April 2001 after they were denied marriage licenses. Clerks in the state's 351 cities and towns have made plans to bring in volunteers and expand their work space in anticipation of a deluge of couples.

Opponents of gay marriage planned protests Monday and promise to continue to fight the state high court ruling and to pursue state and federal amendments banning gay marriage.

Out-of-state gay couples, meanwhile, are likely to challenge the state's 1913 marriage statute, which Gov. Mitt Romney, a gay-marriage opponent, has cited to limit marriages to only Massachusetts residents. The law, which gay-rights advocates have labeled discriminatory, bars out-of-state couples from marrying in Massachusetts if the union would be illegal in their home state.

Several local officials, including those in Provincetown, Worcester and Somerville, have said they will not enforce Romney's order and will give licenses to any couples who ask, as long as they sign the customary affidavit attesting that they know of no impediment to their marriage.

In Provincetown, visitors were greeted with a sign that read "Congratulations, newlyweds!"

"It's the next evolution in the history of marriage," said John Yarbrough of Minnesota, who traveled to Provincetown to marry his partner, Cody Rogahn. "The idea of who you love shouldn't be dictated by the government."