The California Supreme Court today upheld Proposition 8, the voter-approved law restoring a ban on same-sex marriages in the state, but at the same time left intact the more than 18,000 marriages for gay and lesbian couples who wed last year before the ballot measure went into effect.

The Supreme Court’s decision puts California in unusual territory for the time being, establishing a two-tiered system of marriage across the state for same-sex couples. Under the ruling, Proposition 8 will continue to outlaw same-sex marriage in the future, but those gay and lesbian couples who got their marriage licenses before last November’s election will remain on equal legal footing with heterosexual couples.

At least 2,000 “Marriage Equality” supporters gathered in front of the Supreme Court this morning to await the ruling, including Zinnia Gaines and her wife, Rasheeda Gaines. The African American couple got married Sept. 25, 2008, just a few months before the November election. As word of the decision spread, the crowd let out a resounding “boo” and many hugged. Zinnia, who was holding a large poster that included a copy of their marriage certificate, photographs from their wedding and of their 7-year-old son, burst into tears.

“They keep talking about gays and lesbians as if we are not humans,” said Zinnia Gaines. “Even though we got married before the election, it’s still scary because they can always revote.”

Gaines said she feared that Prop. 8 supporters could somehow file another statewide ballot initiative that could ultimately nullify her marriage.

“We have to really start getting out there and telling the stories of our families, and letting our children talk,” Zinnia Gaines said.

After the court’s decision was released, hundreds of people somberly walked across the plaza to San Francisco’s City Hall, where the first gay marriages were performed in February 2004. Inside, attorneys for several organizations that fought Prop 8 gave an hour long news conference.

“It is impossible to square the elation we felt a year ago with the grief we feel today,” said Kate Kendall, executive director of the National Center for Lesbian Rights. “It is clear this is not the end.”

Gay marriage advocates vowed to take the fight to “win marriage back” to California voters again in 2010.

About 150 protesters, some holding roses, blocked the intersection at Van Ness Avenue and Grove Street near city hall with hundreds more rallying around them. Police stood nearby to contain the crowd but kept traffic away until noon when they began arresting people still blocking the intersection.

The 6-1 decision to uphold Prop 8 was widely expected by legal experts, as it was considered unlikely the justices would have the legal authority to overturn a voter-approved amendment to the California constitution. Only Justice Carlos Moreno voted to strike down Prop 8.

Chief Justice Ronald George, who authored last year’s ruling striking down the state’s prior ban on gay marriage, wrote today’s majority opinion upholding Prop 8, cautioning that the decision is not based on whether the measure “is wise or sound as a matter of policy,” but instead “concerns the scope of the right of the people … to change or alter the state Constitution itself.”

The ruling is likely to shift the battleground over gay marriage back to the political arena, as gay rights advocates already are mobilizing to push another ballot measure to erase Prop 8, approved by voters by a 52 to 48 percent margin.

Civil rights groups are hopeful they can duplicate the political momentum they’ve gained in Iowa and on the East Coast, where a number of states have moved in recent months to legalize gay marriage.

“While we were hoping the court would rule in favor of equality, we have been building the infrastructure to win marriage equality rights at the ballot box,” said Rick Jacobs, chair of the pro-gay marriage Courage Campaign.

Gay marriage foes quickly praised the ruling.

“In America, we respect the results of fair elections,” said Austin Nimocks, senior legal counsel for the conservative Alliance Defense Fund. “The California Supreme Court arrived at the only correct conclusion available.”

In a statement, Gov. Arnold Schwarzenegger repeated his past stance that he believes gay marriage willl eventually be permitted in California. “While I believe that one day either the people or courts will recognize gay marriage, as governor of California I will uphold the decision,” he said, adding the justices “made the right decision” to keep the existing marriages intact.

For the state Supreme Court, today’s ruling rested on very different issues than what the justices considered last May, when they overturned California’s previous ban on gay marriage in a historic ruling that rocked the state and sent thousands of same-sex couples scrambling to the altar. In that 4-3 decision, the court concluded that a ballot measure and family law statute outlawing same-sex marriage violated the California constitution’s equal protection guarantees for gays and lesbians by depriving them of the equal right to marry.

But Prop 8 altered the legal debate because it actually amended the state constitution itself, the ultimate trump card against Supreme Court intervention. The majority portrayed Prop 8 as a measure that has limits, removing the label of marriage for same-sex couples but not “the right of those couples to establish an officially recognized family relationship.”

Among other things, Prop 8 does not impact the state’s strong domestic partnership protections for same-sex couples. Gay rights advocates, however, have decried such a system because it sets up a separate status from heterosexual couples.

Karen Strauss and Ruth Borenstein, lead plaintiffs in one of the legal challenges to Prop 8, expressed disappointment that they will be unable to marry now. “I was so hoping the court would find its way to a decision that continues, rather than repeals, our equality under the law,” Strauss said.

In his dissent, Moreno, who’d been mentioned in recent weeks as a possible U.S. Supreme Court candidate, warned that the ruling “places at risk the state constitutional rights of all disfavored minorities.”

A number of local governments, including San Francisco and Santa Clara County, challenged the measure, along with civil rights groups and same-sex couples seeking the right to marry. The central argument was that Prop 8 amounted to an improper method of amending the California constitution, and that it unfairly targeted a minority group by taking away the right to marry.

Attorney General Jerry Brown went further, arguing that Prop 8 should be invalidated because it conflicted directly with last year’s state Supreme Court ruling finding a gay marriage ban unconstitutional.

Prop 8 supporters defended the law, arguing that the Supreme Court should not tamper with a voter-approved amendment to the constitution. Kenneth Starr, the former Whitewater special prosecutor and now dean of Pepperdine University law school, led the defense of the ballot measure in court.

Mercury News Staff Writers Dana Hull and Sean Webby contributed to this report. Contact Howard Mintz at hmintz@mercurynews.com or (408)-286-0236