Italian police on Friday arrested four alleged hackers believed to belong to the activist group Anonymous for attacking websites, including those of the Vatican and the parliament in Rome.

The four have been placed under house arrest and communications police carried out a dozen raids.

They are suspected of launching computer attacks on "strategic and institutional infrastructure" -- often for their own illicit personal ends.

There was a huge reaction on social media against the police operation dubbed "Tango Down".

"The web is a fundamental environment for the evolution of democracy, of information and for the forming of public opinion but precisely because of this we have to be careful and intervene against people who use it for illegal ends," said Filippo Bubbico, a junior interior minister.

"This is an important operation," he said.