The FBI yesterday issued more than 40 search warrants across the US as part of its Anonymous probe, where the distributed denial of services (DDoS) attacks the group carried out on the websites of companies including MasterCard and Visa are punishable by up to 10 years in prison.
Last night Anonymous issued a statement branding the UK arrests "a serious declaration of war" against the group of internet "hacktivists".
Yesterday's arrests are the first in the Metropolitan police's central e-crime unit investigation in the UK.
Three teenagers, aged 15, 16 and 19, were held along with two others in a series of raids in the West Midlands, Northamptonshire, Hertfordshire, Surrey and London early yesterday. The teenagers have to return to their local police station on 13 April and the two men the following day.
Chris Wood, the 20-year-old Anonymous member who spokeextensivelyto the Guardian under his online alias Coldblood last month, is understood to be one of those arrested in yesterday's raids.
Police are said to have seized Wood's computers, mobile phones, hard drives and other storage devices in the arrest, and did not disclose when they would be returned.
The five were arrested in connection with the thousand-strong group known as Anonymous, which last month launched a series of crippling attacks on the websites of companies that had withdrawn support for WikiLeaks, along with a number of government sites in Tunisia and Egypt.
DDoS attacks, which bring down sites by bombarding them with repeated requests to load webpages, are illegal in the UK under the Computer Misuse Act and carry a maximum penalty of 10 years in prison and a £5,000 fine.
"You can easily arrest individuals, but you cannot arrest an ideology. We are united by a common objective and we can and will cross any borders to achieve that," Anonymous said in its statement.
"So our advice to you, the UK government, is to take this statement as a serious warning from the citizens of the world. We will not rest until our fellow anon protesters have been released."
A Europe-wide investigation being carried out in conjunction with Scotland Yard has so far led to two Dutch suspects being arrested and subsequently released.
The group is understood to have grown significantly in number and firepower since its support of WikiLeaks, with the overwhelming majority of users simply volunteering their computer to be used in the attacks.
Most of those involved in Anonymous operations do not disguise their internet protocol (IP) address, meaning they can be easily identified by police authorities.