WASHINGTON — The Supreme Court heard arguments Tuesday over whether a federal law to prevent children under 17 from accessing sexually explicit material on the Internet violates the free speech rights of Web page designers and adult computer users.

Bush administration lawyer Theodore Olson defended the law’s constitutionality, calling it necessary to shield impressionable youngsters from the “menace” of Internet pornography.

But an attorney for the American Civil Liberties Union, Ann Beeson, called the Child Online Protection Act unconstitutionally broad by making it a crime to transmit explicit images and making it more difficult for adults to access “adult” Web sites.

The justices are seeking to balance the government’s interest in protecting children in cyberspace and the First Amendment right of adults to freely send and receive non-obscene pictures and messages on the Internet. The court has engaged in this balancing act many times before, such as when it has allowed strip clubs to operate as long as they are located far from schools and parks.

But the Internet is different and potentially more perplexing for the court. Computers, unlike strip clubs, are quickly becoming as ubiquitous as televisions. And children are often more skilled than adults at logging on to the Internet and accessing its many sexually explicit sites, a modern reality not lost on the justices.

“There’s all kinds of stuff out there” online, said Justice Sandra Day O’Connor, widely regarded as holding the potential swing vote on free speech matters. The justices are expected to render their decision on the law’s constitutionality by July.

A lower federal court already has struck down the law, saying it violates the free speech rights of adults by broadly prohibiting the knowing transmission of “indecent” messages to minors and displaying “patently offensive” sexually explicit images in a manner available to children.

The law’s prohibitions are also unconstitutionally vague and overbroad, making it difficult for page designers to know which of their posted images might be deemed illegal, the Philadelphia-based 3rd U.S. Circuit Court of Appeals ruled.

Appealing that decision to the Supreme Court, Olson argued that the law is a legitimate federal response to the ballooning number of easily accessible pornographic sites. Olson said he found a list of 6 million such sites by conducting an Internet search using the key words “free porn.”###

Olson added that the law is more effective than commercial software programs designed to filter out sexually explicit sites. An Internet search for “disable filter” yielded a list of sites that children can access and which provide programs that render the blocking systems useless, he said.

While parents should keep track of their children’s Internet usage, the Constitution permits Congress to take limited steps “independent” of mom and dad to protect minors, argued Olson, the U.S. solicitor general and President Bush’s chief advocate before the justices.

The law imposes a criminal fine of up to $50,000 and six months in prison for people who post sexually explicit images on the Internet they know can be accessed by minors. The person can also face a civil fine of $50,000 per violation