Detectives from the Met's Police Central e-Crime Unit (PCeU) arrested the five males - aged between 15 and 26 - in connection with offences under the Computer Misuse Act 1990.

The DDoS (distributed denial of service) attacks were carried out late last year by a loose collective called Anonymous in a campaign, dubbed 'Operation Payback', against the sites of 'anti WikiLeaks' companies.

MasterCard, PayPal, Visa and Moneybookers were all targeted for refusing to process payments for the whistle-blowing Web site.

The PCeU says the five arrested males remain in custody as part of an ongoing investigation into Anonymous being carried out in conjunction with agencies in Europe and the US.

The Met's action was mirrored in the US, where the FBI executed more than 40 search warrants across the country and warned that anyone found guilty of executing a DDoS attack faced up to 10 years in prison.

In December, authorities in Holland arrested a teen in connection with the Anonymous assaults. Within hours of the news going public, the Web sites of police and prosecutors in the Netherlands also began to crumble.

Separately, a Norwegian firm asked by Visa to investigate WikiLeaks' fundraising has found no evidence that it is breaking the law in its base of Iceland, according to the Associated Press.

The financial services firm Teller found no proof that WikiLeaks' Sunshine Press fund-raising arm has contravened either Visa's rules or Iceland's legislation, says the AP, citing a document.

However, Visa Europe says it will continue to block all donations to the controversial site until it has completed its own investigation, already in its eighth week.