A voter-approved law prohibiting sex offenders from living within 2,000 feet of a school or a park amounts to additional punishment for the offenders' original crimes, a state appeals court has ruled in a case that could affect thousands of parolees.

The ruling Wednesday by the Fourth District Court of Appeal in Santa Ana was the first by a California court to find that the residency restrictions in Proposition 83, a November 2006 initiative, are not just public safety measures but also would punish ex-offenders by forcing them out of their homes.

Prop. 83, called Jessica's Law by its sponsors, imposes "traditional banishment under another name," the court said.

The ruling leaves the law in effect but could limit its application. The U.S. Constitution forbids laws that retroactively impose criminal penalties or increase punishment for past offenses.

A lawyer for four men who are challenging Prop. 83 before the state Supreme Court said Thursday that the ruling should prevent the state from imposing the residency restrictions on parolees who committed sex crimes before the ballot measure passed.

The attorney, Ernest Galvan, said the state now is applying the 2,000-foot buffer zone requirement to any former sex offender who has been paroled since Prop. 83 passed, even if the parolee committed a sex crime many years earlier and was serving a sentence for an unrelated crime. He said at least 2,000 parolees fall into that category, and the number is growing by hundreds each month.

"You can't criminalize conduct after it's already happened, can't increase the punishment, because everyone's entitled to notice of what's criminal now," Galvan said.

Lisa Page, spokeswoman for Gov. Arnold Schwarzenegger, said the administration is reviewing the ruling. "The governor strongly supports Jessica's Law and keeping families and children safe," she said.

Prop. 83, approved by 70 percent of the voters, increased sentences for various sex crimes and also barred all registered sex offenders - whose crimes range from forcible rape to indecent exposure - from living within 2,000 feet of a school or park where children regularly gather.

State law previously prohibited only convicted child molesters from living within a quarter-mile of a school. Prop. 83 makes most densely populated areas of California off limits to paroled sex criminals, including nearly all of San Francisco.

The state initially sought to apply the residency restrictions to all 90,000 registered sex offenders in California, but federal judges ruled that it did not cover anyone paroled before Prop. 83 passed.

In the case pending before the state Supreme Court, the four parolees say the residency restrictions are an unconstitutional retroactive punishment and an unreasonable parole condition. They say the limits should be applied only to those who committed crimes against children.

In Wednesday's ruling, the appeals court overturned the residency restrictions on an Anaheim man, Steven Lloyd Mosley, who was convicted in 2007 of assaulting a 12-year-old girl four years earlier but acquitted of committing a lewd act. The trial judge nevertheless concluded that Mosley had committed a sex crime, ordered him to register with police as a sex offender and barred him from living within 2,000 feet of a school or park.

In a 3-0 ruling, the court said the judge's order would have been valid if the Prop. 83 requirements were simply nonpunitive measures to protect the public. But because they have an "overwhelmingly punitive effect," the court said, they can be imposed only after a jury trial and conviction for a sex crime. That also means the restrictions can't be applied retroactively.

The law forces many parolees to leave their homes and leaves them under "constant threat of ouster" from their new residences if a new school or park opens nearby, said Justice Raymond Ikola.