Gov. Chris Gregoire signed into law a measure that makes Washington state the seventh to legalize same-sex marriage, but opponents almost immediately filed a referendum to challenge the new law, meaning voters likely will have the final say.

Gregoire signed the bill Monday, saying it was “a day historians will mark as a milestone for equal rights, a day when we did what was right, we did what was just, and we did what was fair.”

The law takes effect June 7, but opponents on multiple fronts already are preparing to fight.

A group called Preserve Marriage Washington filed Referendum 73 Monday afternoon. If they collect the more than 120,577 valid voter signatures needed by June 6, the law will be put on hold pending the outcome of a November vote. Separately, an initiative was filed at the beginning of the legislative session that opponents of gay marriage say could also lead to the new law being overturned.

“I think in the end, people are going to preserve marriage,” said Joe Fuiten, senior pastor at Cedar Park Church in Bothell who is involved in the referendum effort.

The Washington, D.C.-based National Organization for Marriage, which was involved in ballot measures that overturned same-sex marriage in California and Maine, has promised to work with Preserve Marriage Washington to qualify the referendum to overturn the new law.

A campaign has already formed to fight any challenge to the new law. “Washington United for Marriage,” a coalition of gay marriage supporters, formed in November to lobby the Legislature to pass the measure and to run a campaign against any referendum challenging it.

Gay marriage supporters said that while they are ready for a campaign battle, they are allowing themselves to celebrate first.

“You have to relish this moment,” said 31-year-old Bret Tiderman of Seattle, who attended Monday’s bill signing.

The state reception room at the Capitol was packed with hundreds of gay rights supporters and at least 40 lawmakers from the House and Senate to watch Gregoire sign the bill Monday. Gregoire was greeted with loud cheers.

“No matter what the future holds, nothing will take this moment in history away from us,” Sen. Ed Murray, a Seattle Democrat who is gay and has sponsored gay rights legislation for years, told the cheering crowd.