US data security company Imperva says downloads of the attack programme used to bombard websites with bogus requests for data have jumped to over 40,000, with thousands of new downloads reported yesterday .

The freely-available software, dubbed Low Orbit Ion Cannon, is a critical part of the campaign by “hacktivists” seeking to take revenge on sites they believe have betrayed WikiLeaks, which has outraged American officials by publishing hundreds of thousands of classified US diplomatic cables.

Users who download the software essentially volunteer their computers to be used as weapons that volley streams of electronic spam at targeted websites. The more computers, the better chances are of overwhelming the targeted website.

The cyberguerillas, who gather under the name Anonymous, have had mixed results.

Attacks directed at the main pages of Visa and MasterCard succeeded in making them inaccessible, in MasterCard’s case for several hours.

Attacks on online payment company PayPal have periodically rendered a small part of its website inoperative.

But other planned attacks on London-based Moneybookers.com or Amazon.com have either fizzled out or been called off.

All five sites have severed their links to WikiLeaks recently.

The moves angered WikiLeaks’ supporters and alarmed free speech advocates, many of whom claim the companies are caving in to US pressure to muzzle the website.

WikiLeaks distanced itself from Anonymous, saying “we neither condemn nor applaud these attacks”.

A press release under the Anonymous name said the group — which it refers to as an “internet gathering” — wanted “to raise awareness about WikiLeaks and the underhanded methods employed by the above companies to impair WikiLeaks’ ability to function”.

Imperva said it had monitored Anonymous supporters boasting about bringing in huge numbers of extra computers to back the attacks — something it said might challenge Amazon.com at one of the retailer’s busiest times of the year.

But Imperva web researcher Tal Be’ery stressed the boasts were unconfirmed, and the Anonymous statement said its members did not want to alienate the public by causing online havoc over the holidays.

“Attacking a major online retailer when people are buying presents for their loved ones would be in bad taste,” the press release said.

Dutch police said yesterday they were investigating whether hackers were responsible for taking down the websites of police and prosecutors in the Netherlands after the arrest of a 16-year-old suspected cybercriminal.

In Australia, WikiLeaks supporters held rallies in Brisbane and Sydney yesterday.

The US Department of Justice, meanwhile, is considering whether to charge those behind the leaks.