U.K. Prime Minister David Cameron said the government is considering blocking social networks and messaging services amid the worst riots since the 1980s, prompting condemnation from lawyers and free speech advocates.

The government is working with police, the intelligence services and companies to look at “whether it would be right to stop people communicating via these websites and services when we know they are plotting violence, disorder and criminality,” Cameron said today in parliament. He mentioned Research In Motion Ltd. (RIM)’s BlackBerry Messenger service as one of the tools that were used by rioters.

Police have said they are investigating the use of social- networking services such as those operated by Twitter Inc., Facebook Inc. and BlackBerry Messenger. Three people were arrested by police in Southampton, England, on suspicion of using social media and messaging to encourage rioting.

“If you try to stop people communicating, you create more of a problem,” said Jim Killock, executive director of the Open Rights Group, an organization promoting freedom of expression on the Internet. “People are angry because their freedoms are threatened.”

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