We need access to information and it's not up to the government to say what that information is

Judith Krug, American Library Association

The groups have gone to court in Pennsylvania, arguing that the Children's Internet Protection Act is a form of censorship and violates the constitutional right to freedom of information.

The BBC's Jacky Rowland in Washington says the latest court case highlights the conflict between the right to free speech and the desire to protect children against obscenity.

Two previous attempts by Congress to restrict access to indecent material on the internet were blocked by legal challenges.

Groups fighting the case say the software designed to filter out pornographic material can still let it through, while inadvertently blocking internet sites which have nothing to do with pornography.

Better education and dialogue is the answer, they say, not government Big Brother tactics.

"Our perspective is that for all of us to govern ourselves effectively, we need access to information and it's not up to the government to say what that information is," Judith Krug of the American Library Association said.

They're still not perfect but neither are safety belts, and we use them

Miriam Moore, Family Research Council

The law - passed by Congress just over a year ago - seeks to prevent children from accessing pornographic internet material by cutting off federal funds to libraries that fail to install filtering software.

At present, schools and libraries in the US get cheap access to the internet thanks to a special education rate, and the critics say the law denies full web access to many poor people for whom the library is the only place to get it.

'Preventive measure'

The government says that the law does not censor libraries, because they can simply decline to accept funding.

It argues that printed pornographic materials are not in a library's collection and there is no reason why they should be online.

Advocates of the law also say that filtering software has vastly improved since the measure was adopted.

"They're still not perfect but neither are safety belts, and we use them. It's a preventive measure," Miriam Moore of the Family Research Council said.