Hackers have temporarily knocked offline a website run by the British police Serious Organised Crime Agency (SOCA), which targets organised crime in Britain and overseas.
Lulz Security, a loosely aligned hacker group which said it brought down the SOCA Website on Monday, has gone after a long list of government and corporate websites in the past month.
The SOCA incident was likely a denial-of-service attack where Lulz hackers bombarded the site with so many messages that it went offline.
"We are aware of claims that the SOCA Website has been attacked. The picture is not clear at this time but we are investigating the matter with our service provider," said SOCA spokesman Richard Sellors.
Lulz also hacked into a U.S. Senate server, and claimed responsibility for temporarily knocking offline the CIA's public website.
In a posting on Sunday, Lulz Security declared that the "Lulz Lizard battle fleet is now declaring immediate and unremitting war" on government and whitehat security.
As part of that, Lulz, which derives its name from the plural variant of Internet slang for "laugh out loud," urged its followers to hack into and deface government websites.
"Top priority is to steal and leak any classified government information, including e-mail spools and documentation. Prime targets are banks and other high-ranking establishments," Lulz said in the statement on Sunday.
Lulz said it was working with Anonymous, a second international group of hackers.
The groups' stated goals have been murky. In the past, Anonymous has sought to support Julian Assange and Bradley Manning, who face charges after releasing U.S. government documents as part of Wikileaks.
Lulz has also sought to punish Sony for failing to secure data, but did so by releasing the data of Sony customers, exposing them to potential identity theft.
(Additional reporting by Georgina Prodhan in London; Editing by Gunna Dickson).