Hundreds of people on both sides of the gay marriage debate rallied outside the Statehouse Wednesday as lawmakers began working their way through a stack of proposed constitutional amendments, including one that would define marriage as the union of a man and woman.

Demonstrators held signs, waved banners, sang songs and urged passing motorists to honk in support of their cause.

"I think this is an issue for the people to decide," said Jonathan Gal, 39, of Lexington, wearing a sticker that read "Support One Man, One Woman." "I don't like the way this is being imposed on us by a small minority — the courts and the Legislature."

Across the street, supporters of same-sex unions cast the issue as one of civil rights.

"When does civil rights get put on the ballot for everyone to vote on?" said Jim Singletary, 44, of Salem, who last year married his longtime partner, Jim Maynard.

"This is for fairness for my family," Maynard said.

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The goal of the gay marriage amendment, which supporters hope to put on the 2008 ballot, would be to block future gay marriages in Massachusetts. More than 8,000 same-sex couples have taken vows since gay marriages began in May 2004.

Minutes after Senate President Robert Travaglini opened the constitutional convention, there was an unsuccessful attempt to change the order of the agenda, moving up the gay marriage proposal to guarantee debate before the clock ran out on the one-day session. A unanimous vote was needed to make the change.

Travaglini said he intended to get to all of the 20 proposed amendments on the calendar but there was no guarantee they would reach the marriage proposal, which was near the bottom of the stack.

If lawmakers wanted to continue debate past 9 p.m., they would have to vote to do so, and they would have to vote again to go past midnight. It's up to Travaglini to decide when the convention would reconvene, which could happen any time before the end of the year.

To get on the ballot, the question must twice win the backing of 25 percent — or 50 — of the state's 200 lawmakers: once during the current session and again during the session starting in January.

The lawmakers started the session with arguments on a proposed amendment that would require Massachusetts to guarantee every resident access to affordable health care, including mental health care and prescription drugs.

Crowds of gay marriage supporters and foes packed the balcony of the House chamber.

The debate comes less than a week after New York's highest court rejected same-sex couples' bid to win marriage rights and Georgia's high court reinstated that state's constitutional amendment banning gay marriage.

Gay marriage opponents in Massachusetts got a boost this week from an unlikely ally — the Supreme Judicial Court, the same court that handed down the historic ruling legalizing gay marriage.

On Monday, the court ruled that the proposed amendment could go forward, provided it clear the remaining legislative hurdles. Gay marriage supporters had sued to block the question.

It's not the first time Massachusetts lawmakers have been confronted with the issue. In 2002 opponents of gay marriage tried to place a similar constitutional amendment on the ballot, but lawmakers voted to adjourn rather than vote on the issue.

Lawmakers again addressed the issue after the 2003 court ruling that legalized gay marriage. The Legislature approved an proposed amendment banning gay marriage and legalizing Vermont-style civil unions. They later reversed themselves.