Anonymous, the "hactivist" collective behind a series of pro-WikiLeaks cyber attacks, has declared war on the British Government following the arrest of five of its members in the UK.

In a statement posted online, the organisation urged supporters to hit Government websites with distributed denial of service (DDoS) attacks – a way of flooding a target website with so many requests for information that it is forced to shut down.

The tactic has been used by Anonymous activists with remarkable success in the past two months to temporarily disable the websites of financial institutions or governments which have criticised WikiLeaks in the wake of the publication of thousands of secret US embassy cables.

The call to arms raises the spectre of co-ordinated cyber attacks on Government online infrastructure. It has been judged serious enough for GovCertUK, the information security agency, to warn government websites to take precautions.

The threat comes after police in Britain and the US ramped up investigations into the cyber protests. Last week the Metropolitan Police arrested five men in connection with pro-WikiLeaks attacks by Anonymous members. In the US, FBI agents conducted 40 raids on suspected Anonymous supporters at the weekend as part of an ongoing probe into recent attacks on Mastercard, Visa, PayPal and Amazon, who all have refused services to WikiLeaks after the cable leaks.

Anonymous said arresting somebody for taking part in a DDoS attack is like arresting them for attending a peaceful demonstration. "This right to peacefully protest is one of the fundamental pillars of any democracy and should not be restricted in any way."