Police said the five males, aged between 15 and 26, are being held after a series of arrests in the West Midlands, Northamptonshire, Hertfordshire, Surrey and London.
Three teenagers, aged 15, 16 and 19, were arrested in a series of coordinated raids at 7am along with two men aged 20 and 26. All five are being held in custody at local police stations.
The five were arrested on suspicion of being involved in the loose-knit group of "hacktivists" known as Anonymous, who temporarily crippled the websites of MasterCard, Visa and PayPal after those companies cut off financial services to WikiLeaks. The attacks followed WikiLeaks' release of US diplomatic cables from late November.
Today's arrests were coordinated by the Metropolitan police working in conjunction with other UK forces and international agencies.
"They are part of an ongoing [Metropolitan police] investigation into Anonymous which began last year following criminal allegations of DDoS [distributed denial of service] attacks by the group against several companies," Scotland Yard said.
"This investigation is being carried out in conjunction with international law enforcement agencies in Europe and the US."
The so-called DDoS attacks, which bring down sites by bombarding them with repeated requests to load web pages, are illegal in the UK under the Computer Misuse Act and carry a maximum fine of £5,000.
Anonymous leapt to the support of WikiLeaks after Amazon and other companies terminated business links with the site. The 1,000-strong group of activists launched what they called Operation Payback, vowing to give perceived anti-WikiLeaks firms a "black eye".
More recently, the group has turned its attention to supporting the political uprisings in Tunisia and Egypt, temporarily disabling access to 10 Tunisian government websites and four Egyptian government sites.
Anonymous says the cyber attacks are in retaliation for government censorship in both countries. Facebook, Twitter and other information-sharing sites have been routinely blocked by the authorities in Tunisia and Egypt.
Ireland's main opposition party's website was also hacked into earlier this month, with Anonymous claiming responsibility for the attack which compromised up to 2,000 people's personal details.
Authorities across Europe signalled an intention to identify those behind the attacks, the majority of which are traceable by their internet protocol (IP) addresses that connect each device to the internet.
The more sophisticated "hacktivists" use technology that makes their connection anonymous on the internet, so authorities and other internet users cannot see who is behind the computer. But most of those involved in the Anonymous attacks are understood to be "average internet citizens", whose location can be discovered through the IP address.
Dutch police last month arrested two teenagers suspected of involvement in the online campaign. They face trial later this year.