SAN FRANCISCO — In a bold political and legal challenge to California law, city authorities officiated at the marriage of a lesbian couple Thursday and said they will issue more gay marriage licenses.

The act of civil disobedience — coordinated by Mayor Gavin Newsom and top officials in the city considered the capital of gay America — was intended to outmaneuver a conservative group called Campaign for California Families. The group had planned to go to court Friday to get an injunction blocking a plan by Newsom to issue marriage licenses to gay couples.

Longtime lesbian activists Phyllis Lyon, 79, and Del Martin, 83, were hurriedly issued a license and were married just before noon by City Assessor Mabel Teng in a closed-door civil ceremony at City Hall, mayor's spokesman Peter Ragone said. The two have been a couple for 51 years.

It remains unclear what practical value their marriage license will have, but the symbolism was clear on a day when lawmakers in Massachusetts were debating a constitutional amendment that would ban gay marriage.

Ragone said that beginning at noon, officials would begin issuing marriage licenses to any gay couples applying for one. One lesbian couple had already lined up outside City Hall, one of the women wearing a white wedding dress.

Lyon and Martin said after the brief ceremony that they were going home to rest and did not plan anything to celebrate. The couple seemed proud of what they had done.

"Why shouldn't we" be able to marry? Lyon asked.

Thursday's marriage runs counter to a ballot measure California voters approved in 2000 that defines marriage as a union between a man and a woman.

State lawmakers later passed a domestic partner law that, when it goes into effect in 2005, will offer the most generous protections to gays outside Vermont.

No state legally sanctions gay marriage, though Massachusetts could become the first this spring. The Massachusetts high court has ruled that gays are entitled under the state constitution to marry.

Newsom was not present for the wedding Thursday. The two official witnesses were Kate Kendell, director of the National Center for Lesbian Rights and former city official Roberta Achtenberg.

The Campaign for California Families did not immediately respond to a request for comment.