The U.S. Supreme Court pulled the plug Wednesday on plans for camera coverage of the same-sex marriage trial in San Francisco and said any televising of federal court proceedings should start with a more humdrum case.

The 5-4 majority said Chief U.S. District Judge Vaughn Walker, presiding over the trial on the constitutionality of California's Proposition 8, had not given the public enough time to comment last week before he approved live telecasts to be shown in several courthouses around the country. Walker also approved videotaping for public Internet viewing, in delayed uploads on YouTube.

The ruling permanently bars a broadcast that the high court temporarily blocked just before the trial started Monday. Opponents of Prop. 8, the November 2008 initiative that prohibits same-sex marriage, are suing to overturn the measure as a denial of equal protection of the law.

The federal appeals court in San Francisco approved a pilot program last month allowing cameras at selected civil, nonjury trials, a project clearly designed for the Prop. 8 trial. Walker approved the telecast over the objections of the ballot measure's sponsors, who said their witnesses could face harassment and might refuse to testify. The sponsors appealed to the Supreme Court.

No federal trial in California has ever been shown on TV or the Internet. The Supreme Court, which has refused to televise its own proceedings, said it was not ruling on whether any federal trials could be televised, but the majority justices made their discomfort clear.

The Prop. 8 trial, which involves "issues subject to intense debate in our society ... is not a good one for a pilot program," because the potential for harm is greater in a high-profile case, the court said.

The justices also said they were concerned about the effect on witnesses' testimony, which "may be chilled if broadcast," the court said.

Walker approved the telecast after only a week of public comment, rather than the 30 days that the San Francisco federal court normally requires when it changes its rules. Walker cited his authority to shorten the comment period when immediate needs arise, and said Monday he had received more than 138,000 comments from the public, all but 32 of them favorable.

But the Supreme Court said there was no immediate need to suspend the 30-day requirement because shutting off the cameras would harm neither side in the case.

Opponents of Prop. 8 had supported television coverage, as had news organizations, including Hearst Corp., which owns The Chronicle.

The ruling was issued by Chief Justice John Roberts and Justices Antonin Scalia, Clarence Thomas, Anthony Kennedy and Samuel Alito.

The dissenters were the court's more liberal members, Justices Stephen Breyer, John Paul Stevens, Ruth Bader Ginsburg and Sonia Sotomayor.

Breyer, writing for the dissenters, said the public had ample opportunity to comment, there was no evidence that witnesses would be harmed, and those outside the courthouse were losing an opportunity to view a trial of "great public interest."

Walker had ordered the trial telecast live to the regional appeals court's headquarters at Seventh and Mission streets in San Francisco and courthouses in Pasadena, Seattle, Portland, Ore., and Brooklyn, N.Y.

Wednesday's ruling means the only place people can watch the case on TV is in a 19th floor conference room at the Golden Gate Avenue courthouse that seats about 150. Thirty-six seats are available to the public for the trial itself in Walker's courtroom on the 17th floor.

The case is Hollingsworth vs. Perry, 09A648. The ruling can be viewed at links.sfgate.com/ZJCL.

Day's testimony: Judge looks at whether anti-gay bias played a role in marriage ban's victory. C2