A federal appeals court in San Francisco struck down a major section of the federal law against child pornography yesterday, ruling that the government cannot prohibit computer-generated sexual images that only "appear to be" pictures of children.

Federal law enforcement officials quickly expressed fear that the ruling could significantly hamper their efforts to combat child pornography. And within hours of the court's decision, federal prosecutors announced that because of the ruling they would release Patrick J. Naughton, a former Internet executive with Walt Disney Co.'s Go.com site who was convicted Thursday of possessing child pornography.

Although prosecutors accused Naughton of having pictures of children, they decided to release him on $100,000 bail until the impact of the court's ruling could be sorted out. He is expected to be released on Monday.

The 2 to 1 decision by the 9th U.S. Circuit Court of Appeals, which has jurisdiction over federal courts in California and eight other western states, upheld much of the 1996 Child Pornography Protection Act but ruled that applying it to "virtual pornography" would violate the Constitution.

Judge Donald W. Molloy wrote in the court's opinion that the government has "no compelling interest in regulating sexually explicit materials that do not contain visual images of actual children."

Federal appeals courts in Atlanta and New York have upheld the anti-pornography law, making it likely that the Supreme Court will eventually review the ruling.