PHILADELPHIA — Eight years after Congress passed a law aimed at protecting children from online pornography, free speech advocates and Web site publishers argued in federal court yesterday that the never-enforced measure is fatally flawed.

Salon.com, Nerve.com, and other plaintiffs warned that the 1998 Child Online Protection Act could be used to criminalize such things as sexual health information, erotic literature, and news photographs of naked prisoners tortured at Abu Ghraib.

The law, signed by then-President Clinton, says Web site operators must prevent youngsters from seeing material “harmful to children” by demanding proof of age from computer users. It would impose a $50,000 fine and six-month prison term on commercial Web site operators that allow minors to view such content, which is to be defined by “contemporary community standards.”

In a trial that opened yesterday, the plaintiffs argued that “community standards” is too vague.

“As a parent, I know that what’s fine for my daughter may not be appropriate even for some of her friends,” the editor in chief of Salon.com, Joan Walsh, testified

The law has never been enforced.

The U.S. Supreme Court has twice upheld preliminary injunctions that prevented the government from enforcing the law until a trial to determine the act’s constitutionality can be held.

The American Civil Liberties Union, which represents the plaintiffs, argues that filter programs installed in home computers are more effective ways of policing the Internet.

A government attorney, Eric Beane, acknowledged that it is tempting to defer to families on the question of what is appropriate for children but said the filters used by parents do not work.

“The evidence will show that a shocking amount of pornography slips through to children,” Mr. Beane said in opening statements.