Eight years after Congress tried to criminalize material deemed "harmful to children," free speech advocates and Web site publishers took their challenge of the law to trial Monday.

Salon.com, Nerve.com and other plaintiffs backed by the American Civil Liberties Union are suing over the 1998 Child Online Protection Act. They believe the law could restrict legitimate material they publish online — exposing them to fines or even jail time.

The Justice Department argues that it is easier to stop online pornography at the source than to keep children from viewing it.

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The law, signed by then- President Clinton, requires adults to use some sort of access code, or perhaps a credit-card number, to view material that may be considered "harmful to children."

It would impose a $50,000 fine and six-month prison term on commercial Web site operators that publish such content, which is to be defined by "contemporary community standards."

It has yet to be enforced, however.

The U.S. Supreme Court has twice granted preliminary injunctions, including one in June 2004 in which it ruled 5-4 that the plaintiffs were likely to prevail.

The ACLU argues that filters are a more effective way of policing the Internet. It notes that the law would not regulate any material posted overseas.

The government "will argue that parents are too stupid to use filters. It's an insulting argument and it's wrong," ACLU attorney Chris Hansen said in his opening statement Monday.

Eric Beane, a government attorney, acknowledged that it is tempting to defer to families on the question of what is appropriate for children, but said the patchwork of filters used by parents don't work.

"The evidence will show that a shocking amount of pornography slips through to children," Beane said.

The nonjury trial in front of U.S. District Judge Lowell Reed is expected to take about a month.

The plaintiffs, technology experts and even Supreme Court Justice Anthony M. Kennedy have expressed concerns that the law has already been surpassed by technology and the growth of the Internet.

Kennedy noted, for example, that filters can block Web material posted offshore, but the law cannot control what foreigners post online.

In preparing for its defense of the law, the Justice Department sought internal files from search engine companies and Internet service providers.

Google Inc. (GOOG) refused one such subpoena for 1 million sample queries and 1 million Web addresses in its database, although it primarily cited trade secrets, not privacy issues.