SAN FRANCISCO (AP) - In a monumental victory for the gay rights movement, the California Supreme Court overturned a voter-approved ban on same-sex marriage today in a ruling that would allow same-sex couples in the nation’s biggest state to tie the knot.

The court ruled that domestic partnerships are not a good enough substitute for marriage. In the decision, the justices ruled 4-3 in striking down the ban, saying that lesbian and same-sex couples are entitled to the same fundamental right to marry as heterosexual couples as protected by the California constitution.

The decision explicitly strikes down Proposition 22, a voter passed initiative that passed in 2000 that sought to limit marriage to only between a man and woman. Under California rules of procedure, the decision can not go into effect for at least 30 days.

Outside the courthouse, same-sex marriage supporters cried and cheered as the news spread.

Jeanie Rizzo, one of the plaintiffs, called Pali Cooper, her partner of 19 years, and asked, “Pali, will you marry me?”

“This is a very historic day. This is just such freedom for us,” Rizzo said. “This is a message that says all of us are entitled to human dignity.”

In the Castro, historically a center of the gay community in San Francisco, Tim Oviatt started crying while watching the news on TV.

“I’ve been waiting for this all my life,” he said. “This is a life-affirming moment.”

The city of San Francisco, two dozen gay and lesbian couples and gay rights groups sued in March 2004 after the court halted the monthlong wedding march that took place when Mayor Gavin Newsom opened the doors of City Hall to same-sex marriages.

“Today the California Supreme Court took a giant leap to ensure that everybody — not just in the state of California, but throughout the country — will have equal treatment under the law,” said City Attorney Dennis Herrera, who argued the case for San Francisco.

The challenge for gay rights advocates, however, is not over.

A coalition of religious and social conservative groups is attempting to put a measure on the November ballot that would enshrine laws banning same-sex marriage in the state constitution.

The Secretary of State is expected to rule by the end of June whether the sponsors gathered enough signatures to qualify the marriage amendment, similar to ones enacted in 26 other states.

If voters pass the measure in November, it would trump the court’s decision.

California already offers same-sex couples who register as domestic partners the same legal rights and responsibilities as married spouses, including the right to divorce and to sue for child support.

But, “Our state now recognizes that an individual’s capacity to establish a loving and long-term committed relationship with another person and responsibly to care for and raise children does not depend upon the individual’s sexual orientation,” Chief Justice Ron George wrote for the court’s majority, which also included Justices Joyce Kennard, Kathryn Werdegar and Carlos Moreno.

In a dissenting opinion, Justice Marvin Baxter agreed with many arguments of the majority but said the court overstepped its authority. Changes to marriage laws should be decided by the voters, Baxter wrote. Justices Ming Chin and Carol Corrigan also dissented.

The conservative Alliance Defense Fund says it plans to ask the justices for a stay of their decision until after the fall election, said Glen Lavey, senior counsel for the group.

Gov. Arnold Schwarzenegger, who has twice vetoed legislation that would’ve granted marriage rights to same-sex couples, said in a news release that he respected the court’s decision and “will not support an amendment to the constitution that would overturn this state Supreme Court ruling.”