SAN FRANCISCO, California (CNN) -- California officials began issuing marriage licenses to same-sex couples Monday evening after a state Supreme Court ruling legalizing the ceremonies took effect.

The May 15 ruling took effect at 5:01 p.m. (8:01 p.m. ET) Monday. Gay and lesbian couples had lined up for hours outside county clerk's offices in anticipation of the decision coming into force.

Lesbian rights activists Del Martin, 87, and Phyllis Lyon, 84, were the first same-sex couple to receive a marriage license in San Francisco on Monday, with Mayor Gavin Newsom presiding over their wedding ceremony.

"This is an extraordinary moment in history," Newsom told a cheering, standing-room-only crowd at City Hall. "I think today, marriage as an institution has been strengthened."

The high-profile mayor heated up the debate on same-sex marriages in February 2004, when he ordered city officials to issue marriage licenses to gay and lesbian couples. Martin and Lyon were the first to exchange wedding vows after the order, only to see the ceremony voided later. iReport.com: Share your feelings, photos and videos

"I think it's a wonderful day and I have to thank our mayor for most of it," Lyon said. "I'm very happy and very grateful for all of you."

Clerks expect a much larger number of couples to show up Tuesday morning to complete marriage-license applications that replace "Bride" and "Groom" with "Partner A" and "Partner B." Watch as longtime lesbian couple emerges from wedding »

The decision makes California the nation's second state, after Massachusetts, to legalize same-sex marriage. Four other states allow civil unions.

Robin Tyler and Diane Olson, one of the first couples to obtain a marriage license Monday evening, rushed outside the courthouse in Beverly Hills for their Jewish wedding ceremony. There, they were greeted by a crowd of family, friends, supporters -- and a few opponents protesting the same-sex marriages with signs reading "repent."

"It's not about us," Tyler said. "It's about the entire movement that helped make this day possible."

Critics of the ruling have talked about staging protests against same-sex marriages this week.

Opponents say they've collected enough signatures to call for a statewide vote on a constitutional amendment that would define marriage as between a man and woman. The California secretary of state must verify the signatures before a November vote can be held on the proposed amendment.

Though the law affects only California, it has national implications. "Because we're in a presidential campaign right now, it's going to be a real flash point for the electorate to come together and say this really matters to us," said Charmaine Yoest of the Family Research Council, a conservative Christian group based in Washington that opposes gay marriage.

Meanwhile, the Archdiocese of Los Angeles, California, issued a statement on behalf of seven bishops Monday, saying the Catholic Church "cannot approve of redefining marriage."

Marriage "has a unique place in God's creation, joining a man and a woman in a committed relationship in order to nurture and support the new life for which marriage is intended," the statement said. "The meaning of marriage is deeply rooted in history and culture, and has been shaped considerably by Christian tradition. Its meaning is given, not constructed."

Newsom said the issue "is about civil marriages, not about religious marriages. ... We're not telling religious institutions what to do. They can continue to do what they've done."

In the May 15 ruling, the California Supreme Court struck down the state's ban on same-sex marriage as unconstitutional. The ruling surprised legal experts because the court has a conservative reputation. Six of its seven judges are Republican appointees.

CNN's Ted Rowlands and Carey Bodenheimer contributed to this report.

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