San Francisco lawmakers disappointed committed nudists on Tuesday by narrowly approving a ban on public nakedness despite concerns the measure would undermine the city's reputation as a sanctuary for free expression.

The Board of Supervisors voted 6-5 in favour of a public safety ordinance that prohibits exposed genitals in most public places, including streets, sidewalks and public transit. The law still must pass a final vote and secure Mayor Edwin Lee's signature to take effect early next year.

Supervisor Scott Wiener introduced the ban in response to escalating complaints about a group of men whose bare bodies are on display almost daily in the city's predominantly gay Castro District.

"The Castro, and San Francisco in general, is a place of freedom, expression and acceptance. But freedom, expression and acceptance does not mean anything goes under any circumstances," Wiener said Tuesday. "Our public spaces are for everyone, and as a result it's appropriate to have some minimal standards of behaviour."

Ban unnecessary: critics

Wiener's opponents on the board said a citywide ban was unnecessary and would draw police officers' attention away from bigger problems while undermining San Francisco values like tolerance and appreciation for the offbeat.

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A naked protester sits on the steps of City Hall in San Francisco, Tuesday. Jeff Chiu/Associated Press

"I'm concerned about civil liberties, about free speech, about changing San Francisco's style and how we are as a city," Supervisor John Avalos said. "I cannot and will not bite this apple and I refuse to put on this fig leaf."

To make his point, Avalos showed his colleagues a clip from the 1970 movie version of Joseph Heller's Catch-22. In it, Orson Welles pins a medal on a naked soldier.

"I get emails all the time about people who are upset there are homeless people, and I would be the last person to legislate a solution for people who do not want homeless people in their neighbourhood," Supervisor Christina Olague said.

Wiener countered that it was inappropriate for hard-core nudists to wrap themselves in the mantle of personal liberty.

"I don't agree that having yellow hair is the same as exposing your penis at a busy street corner for hours and hours for everybody to watch as they go by," he said.

Under Wiener's proposal, a first offence would carry a maximum penalty of a $100 fine, but prosecutors would have authority to charge a third violation as a misdemeanour punishable by up to a $500 fine and a year in jail.

Exemptions would be made for participants at permitted street fairs and parades, such as the city's annual gay pride event and the Bay-to-Breakers street run, which often draws participants in costumes or various states of undress.

A federal lawsuit claiming the ban would violate the free speech rights of people who prefer to make a statement by going au naturel was filed last week in case the ordinance passes.