SAN FRANCISCO — The California Supreme Court upheld a voter-approved ban on same-sex marriage Tuesday, but it also decided that the estimated 18,000 gay couples who tied the knot before the law took effect will stay wed.

Demonstrators outside the court yelled, "Shame on you!"

The 6-1 decision written by Chief Justice Ron George rejected an argument by gay-rights activists that the ban revised the California Constitution's equal-protection clause to such a dramatic degree that it first needed the Legislature's approval.

The court said that Californians have a right, through the ballot box, to change their constitution.

"In a sense, petitioners' and the attorney general's complaint is that it is just too easy to amend the California Constitution through the initiative process. But it is not a proper function of this court to curtail that process; we are constitutionally bound to uphold it," the ruling said.

The justices said the 136-page majority ruling does not speak to whether they agree with Proposition 8 or "believe it should be a part of the California Constitution."

They said they were "limited to interpreting and applying the principles and rules embodied in the California Constitution, setting aside our own personal beliefs and values."

The announcement of the decision set off an outcry among a sea of demonstrators who had gathered in front of the San Francisco courthouse awaiting the ruling. Holding signs and many waving rainbow flags, they yelled, "Shame on you." Many people also held hands in a chain around an intersection in an act of protest.

Gay-rights activists immediately promised to resume their fight, saying they would go back to voters as early as next year in a bid to repeal Proposition 8.

The split decision provided some relief for the 18,000 gay couples who married in the brief time same-sex marriage was legal last year, but that wasn't enough to dull the anger over the ruling that banned gay marriage.

"It's not about whether we get to stay married. Our fight is far from over," said Jeannie Rizzo, 62, who was one of the lead plaintiffs along with her wife, Polly Cooper. "I have about 20 years left on this earth, and I'm going to continue to fight for equality every day."

Also in the crowd gathered at City Hall, near the courthouse, were Sharon Papo, 30, and Amber Weiss, 32, who were married on the first day gay marriage was legal last year, June 17.

"We're relieved our marriage was not invalidated, but this is a hollow victory, because there are so many that are not allowed to marry those they love," Weiss said.

"I feel very uncomfortable being in a special class of citizens," Papo said.