A City Council committee gave preliminary approval to a new curfew Wednesday, but added a sunset provision after hearing concerns that the law would be ineffective and violate the rights of minors.

Council will ask the mayor's office to provide data on the curfew's effect before its proposed expiration in December 2013, said Councilwoman Blondell Reynolds Brown, who sponsored the bill on behalf of Mayor Nutter.

The mayor used an aggressive temporary curfew this summer to tamp down roving bands of youths - dubbed "flash mobs" - who were attacking random pedestrians in Center City.

Brown said she was confident the bill would achieve its goals, and the sunset provision would allow Council to review and amend the law.

"That's going to force us to look at what else the city needs to do," Brown said. "The research should drive public policy, always."

Nutter spokesman Mark McDonald said the sunset provision wasn't needed.

"On the other hand, this is very important legislation, and if Council deems it important to revisit the issue in two years, then so be it," he said.

Mary Catherine Roper, attorney for the American Civil Liberties Union of Pennsylvania, warned Council's Public Safety Committee that the curfew was "almost certainly unconstitutional."

"If it is passed, it will invite litigation," she said.

She compared the bill to the controversial police tactic known as stop-and-frisk, which was meant to target the carrying of illegal guns. Civil rights attorneys said stop-and-frisk instead led to officers stopping pedestrians illegally, based solely on race. The city recently settled a lawsuit over the tactic.

"We've already made the point that our police sometimes focus on the identity of people and not what they are doing," Roper said. "And that's not effective policing."

Although the curfew bill was sparked by the flash mobs, Councilman Curtis Jones Jr. said he hoped the law would assist police in neighborhoods plagued by violent juvenile crime.

He asked the administration for data on juvenile crime committed after the curfew hours in the bill.

"I have to balance civil liberties and the constitutional right to live without being murdered in the street," he said.

Deputy Mayor Everett Gillison said the curfew would not lead to widespread arrests of minors. He said police have stopped about 10,000 juveniles this year, but cited just 200 for curfew violations. Most are simply told to go home, he said, and they do.

Jeffrey Nadel, a University of Pennsylvania freshman and president of the National Youth Rights Association of Philadelphia, said he provided Council with four academic studies that showed curfews fail to lower youth violence or victimization.

"If you have people organized to commit violent acts or thefts, they're not going to be deterred by a curfew," he said. When he was 16, Nadel sued to overturn a curfew in West Palm Beach, Fla., near his home in Boca Raton. That case is pending, but Roper said curfews less restrictive than Council's proposal have been struck down in court.

The bill would set distinct curfews for three age groups of minors and would be structured around the school year. Each age group would get an extra hour in the summer.

Parents could be fined from $75 to $500 if their children are caught violating curfew.

Contact staff writer Troy Graham at 215-854-2730 or tgraham@phillynews.com.