Spanish police arrested three suspected members of the 'Anonymous' group on Friday on charges of cyber-attacks against targets including Sony's PlayStation Network, governments, businesses and banks. The Spanish police said that the three arrested "hacktivists" had been involved in a recent attack on Sony's PlayStation Network, as well as cyber-attacks on Spanish banks BBVA and Bankia and Italian energy group Enel SpA.

The arrests are following similar legal proceedings in the United States and Britain.

Anonymous responded by threatening to retaliate for the arrests: "We are Legion, so EXPECT US," the group said on its official Twitter feed.

And the response came on Saturday with an attack by the Anonymous hackers to the Spanish police website, El Mundo said on its website on Sunday. Access to the website of Spain's national police force was blocked for over an hour late on Saturday in a reprisal attack by the Anonymous hackers group, El Mundo said on its website on Sunday.

In late April, Sony revealed that hackers had stolen personal information from the accounts of 77 million users of its online video-games network.

A week later, it said hackers had stolen data from another 25 million users of its computer games system. Sony's PlayStation Network was crippled for a month as the company tried to find and fix the problem.

In related news, the International Monetary Fund became the latest known target of a major cyber attack on Saturday.

The New York Times cited computer experts as saying the IMF's board of directors was told of the attack on Wednesday, though the assault had lasted several months.

The IMF says its website remains fully functional.