The US government could usurp parental control over what children see on the Internet if a new law designed to restrain online pornography goes into effect, companies opposed to the law told a federal court on Tuesday.

In closing arguments before a US district judge, an attorney for 17 Internet companies assailed the Child Online Protection Act (COPA) as a measure that violates the constitutional right to free speech while imposing unreasonable costs on businesses that populate the World Wide Web.

American Civil Liberties Union attorney, Ms Ann Beeson, said the public would be better served if families opted for commercially available filtering software capable of stopping web browsers on home personal computers from visiting pornographic websites.

"It should be up to parents, and not the government, to decide what children should see," she said at the end of five days of hearings.

Enacted by Congress and signed into law by President Clinton last year, COPA requires US commercial website operators to keep children under 18 away from material deemed harmful to minors by employing credit-card registration or age verification systems.

The law reserves stiff criminal and civil penalties for businesses found in violation.

Plaintiffs led by the ACLU hoped to secure a preliminary injunction blocking COPA's enforcement until their legal challenge to the law reaches trial.

The judge is expected to rule on a preliminary injunction by Monday when a temporary restraining order he imposed against COPA in November is set to expire.