NEW YORK — A judge yesterday struck down New York's ban on gay marriage as unconstitutional, paving the way for five same-sex couples to wed and opening the door to a contentious battle in the state.

State Supreme Court Justice Doris Ling-Cohan, however, stayed her ruling for 30 days — expecting there would be appeals. The Supreme Court is New York's trial-level court.

The lawsuit, filed last year in New York City, challenged the state law regulating marriage; several previous challenges failed and are awaiting action from appellate courts.

Lambda Legal Defense Fund, a national gay advocacy group, hailed the decision as a "historic ruling that delivers the state constitution's promise of equality to all New Yorkers," said Susan Sommer, Lambda's lead attorney in the case.

Elsewhere, state supreme courts in Vermont and Massachusetts have ruled that same-sex couples have a constitutional right to form legally recognized relationships, although only in Massachusetts are they called marriages.

In Washington state, two trial courts ruled last year in favor of same-sex marriage, and the state Supreme Court is scheduled to hear an appeal of those rulings March 8. In Oregon, a lower-court ruling in favor of gay marriage was followed by the passage of a state constitutional amendment barring it.

Lambda also has challenged laws in New Jersey, and has joined with other organizations to appeal laws in California.

"Simply put, marriage is viewed by society as the utmost expression of a couple's commitment and love," Ling-Cohan wrote in a 62-page opinion on the New York case. "Plaintiffs may now seek this ultimate expression through a civil marriage."

Gay and heterosexual couples, she said, "are entitled to the same fundamental right to follow their hearts and publicly commit to a lifetime partnership with the person of their choosing. The recognition that this fundamental right applies equally to same-sex couples cannot legitimately be said to harm anyone."

Opponents of same-sex marriage blasted the ruling, predicting a court battle and potential action by the Legislature to prevent gay unions.

"I believe it is inconsistent with logic and reason," said state Sen. Serpphin Maltese, a Republican who has introduced a bill to ban gay marriage. In a statement on his Web site, Maltese said the ruling ignored "our founding fathers' intention that marriage be an institution between a man and a woman."

New York Attorney General Eliot Spitzer last year issued an opinion that state law banned same-sex marriages; however, he said the law raised "important constitutional questions" and could be subject to legal challenge. He also said same-sex marriage licenses granted in other states should be recognized in New York.

Yesterday's ruling — which for now applies only to New York City — could make its way to the New York Court of Appeals, the state's highest court, legal experts said. Resolution could be months, if not years, away. If the decision is upheld, it could apply statewide.

Details on laws in other states were provided by The Washington Post.