UPDATE 1-UK police arrest WikiLeaks backers for Web attacks
* Five young men held in Britain for cyber attacks
* "Anonymous" group targeted "enemies" of WikiLeaks
* FBI conducts dozens of searches in United States
(Adds FBI searches in United States)
LONDON, Jan 27 (Reuters) - British police arrested five young men on Thursday as they and U.S. authorities conducted searches as part of a probe into Internet activists who carried out cyber attacks against groups they viewed as enemies of the WikiLeaks website.
"The arrests were related to recent 'distributed denial of service' (DDoS) attacks by an online group calling themselves Anonymous," London police said in a statement.
In the United States, the Federal Bureau of Investigation said that agents executed more than 40 search warrants as part of its investigation and that the attacks were facilitated by software the group made available for free on the Internet.
"The FBI is working closely with its international law enforcement partners and others to mitigate these threats," the agency said in a statement, adding that there were other, unspecified investigative and enforcement actions in the Netherlands, Germany and France.
WikiLeaks, which was founded by Australian-born Julian Assange, has disclosed classified U.S. diplomatic dispatches which included candid and embarrassing assessments of world leaders as well as classified documents related to the wars in Iraq and Afghanistan.
In addition to the probe into the cyber attacks, U.S. authorities have been investigating the leak of the documents themselves and their prime suspect has been a former U.S. Army intelligence analyst, Bradley Manning.
Internet activists last month carried out a series of online assaults against institutions they viewed as enemies of WikiLeaks, temporarily bringing down the websites of credit-card giants Visa (V.N) and MasterCard (MA.N), Amazon.com (AMZN.O) and of the Swedish government.
Sweden wants Assange extradited from Britain so he can answer questions over sexual assault allegations.
Officers from a specialist London police unit dealing with online crime detained the five males, aged from 15 to 26, in raids at homes in central and southern England.
Dutch police last month arrested two teenagers suspected of involvement in the online campaign. They face trial later this year.
A DDoS attack consists of swamping the resources of a computer such as a server to make it unavailable to users.
The maximum penalty in Britain for offences of computer misuse is 10 years imprisonment and a fine of 5,000 pounds ($7,953).
In the United States, such a cyber attack carries a maximum punishment of 10 years in prison and significant fines.
(Reporting by Keith Weir in London with Jeremy Pelofsky and James Vicini in Washington; Editing by Elizabeth Fullerton and Eric Walsh)