SAN FRANCISCO -- The San Francisco Board of Supervisors Tuesday approved legislation to amend the city's sanctuary policy to allow city employees to refer undocumented youth accused of serious crimes to federal immigration authorities only upon conviction.

The move prompted cheers from immigration advocates as Mayor Gavin Newsom's office warned it could threaten San Francisco's status as a sanctuary city.

The sanctuary ordinance prohibits city employees from assisting federal immigration authorities, with exceptions for those booked for felony crimes.

The board, as expected, passed Supervisor David Campos' legislation by an 8-2 margin. Supervisor Michela Alioto-Pier, who voted against it in committee, was attending a funeral and excused from Tuesday's meeting.

"We're trying to protect the basic principle that in this country, you are innocent until proven guilty," Campos told board members.

Campos said his legislation "tries to strike a balance between two extremes," between never reporting youth who commit serious crimes and reporting them before they've been found guilty, he said.

The amendment would mandate that youth accused of felonies and suspected of being undocumented be referred by the city to U.S. Immigration and Customs Enforcement only after conviction and not arrest.

Newsom altered the sanctuary policy last year after it was revealed the city wasn't reporting juveniles arrested for felonies, a move he said was needed to bring the city into compliance with state and federal laws. Since then, the juvenile probation department has been referring them at booking.

Immigrant rights groups have argued the current policy denies juveniles due process in court, threatens to break up immigrant families, and encourages racial profiling.

The crowd at Tuesday's meeting, buoyed by students from Lincoln and Mission high schools, erupted in cheers after the vote. Jubilation ensued outside the board chamber, with chants of "Yes we can" and "Si se puede."

Campos joined the group outside, telling them, "This is really for our youth."

"To be undocumented doesn't mean you're not a person under the United States Constitution," Campos said.

Newsom has promised to veto Campos' legislation, but the eight supervisors who approved it Tuesday would be enough to overturn the veto.

"It's the wrong policy," Newsom spokesman Nathan Ballard said after Tuesday's vote. "A sanctuary city should not be used as a shield for criminal behavior."

Ballard reiterated the claim that the new law would be "unenforceable," citing a city attorney's office opinion that it would force city employees to violate federal law. An Aug. 18 memo from City Attorney Dennis Herrera's office warned the legislation could put the city at risk for legal action.

The memo advised that "until further clarification by the federal courts, federal law prohibits the city from taking any adverse action against a city official or employee who reports a juvenile to federal immigration authorities."

"David Campos is taunting the opponents of sanctuary city to file lawsuits against us," Ballard asserted.

If the mayor's veto is overturned, "We'll work with the city attorney to ensure that we don't put our law enforcement officials in the untenable position of becoming lawbreakers," Ballard said.