SAN FRANCISCO (Reuters) - A group opposed to male circumcision said on Tuesday they have collected more than enough signatures to qualify a proposal to ban the practice in San Francisco as a ballot measure for November elections.

But legal experts said that even if it were approved by a majority of the city's voters, such a measure would almost certainly face a legal challenge as an unconstitutional infringement on freedom of religion.

Circumcision is a ritual obligation for infant Jewish boys, and is also a common rite among Muslims, who account for the largest share of circumcised men worldwide.

The leading proponent of a ban, Lloyd Schofield, 59, acknowledged circumcision is widely socially accepted but he said it should still be outlawed.

"It's excruciatingly painful and permanently damaging surgery that's forced on men when they're at their weakest and most vulnerable," he told Reuters.

His group submitted about 12,000 signatures supporting his proposed ban, said Rachel Gosiengfiao, campaign services manager for the city's Department of Elections. The agency has 30 days to verify the petitions. He needs 7,200 valid signatures to qualify.

The measure, which would only apply in San Francisco, would make it a misdemeanor crime to circumcise a boy before he is 18 years of age, regardless of the parents' religious beliefs. The maximum penalty would be a year in jail and a $1,000 fine.

Schofield, who would not discuss his current occupation but previously worked for hotels in the San Francisco Bay area, has found allies for his cause in the anti-circumcision groups Intact America and the National Organization of Circumcision Information Resource Centers, according to his group's website.

However, some experts said it was doubtful such a measure would withstand legal scrutiny if challenged.

"The practice of Judaism requires a boy to be circumcised. I suspect the California courts would ultimately require the city to demonstrate the practice is harmful," said Jennifer Rothman, a professor at Loyola Law School in Los Angeles.

"I don't think there's sufficient medical evidence that it is, which would place the law's constitutionality in question."

But Josh Davis, professor and associate dean for faculty scholarship at the University of San Francisco School of Law, said the U.S. Supreme Court has previously indicated in rulings that "religions don't get a free pass."

"So if circumcision is the harm that's being targeted -- because circumcision is perceived as causing harm, and not because it is a religious practice -- it might well be a constitutionally valid law," he said.

Schofield's proposal would make exceptions for boys who need a circumcision for health reasons.

Nevertheless, Davis and Rothman both said voters would be likely to reject the measure at the polls.

"I think that people are very likely to react to it as interfering with religious practices," Davis said.

(Editing by Alex Dobuzinskis; Steve Gorman and Jerry Norton)