LONDON - WikiLeaks supporters on Friday downloaded increasing amounts of the spam-shooting software used to attack companies seen as hostile, a development that could challenge even Internet giants such as PayPal and Amazon.com during the crucial Christmas shopping season.

U.S. data security company Imperva says downloads of the attack program used to bombard websites with bogus requests for data have jumped to more than 40,000, with thousands of new downloads reported overnight.

"It's definitely increasing," Imperva Web researcher Tal Be'ery said.

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 U.S. diplomatic cables and military intelligence reports.

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 greater the flow of data requests, and 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 Inc. and MasterCard Inc. succeeded in making them inaccessible, in MasterCard's case for several hours. Attacks on online payment company PayPal Inc. 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 or been called off. All five sites have severed their links to WikiLeaks over the last weeks and months, many citing suspected "terms of use" violations.

The moves angered WikiLeaks supporters and alarmed free speech advocates, many of whom claim that the companies are caving in to U.S. pressure to muzzle the controversial website. WikiLeaks has been careful to distance itself from Anonymous, saying "we neither condemn nor applaud these attacks."

A news release circulated under the Anonymous name Friday said the group was acting out of a desire "to raise awareness about WikiLeaks and the underhanded methods employed by the above companies to impair WikiLeaks' ability to function."

Imperva said Friday that it had monitored Anonymous supporters boasting about bringing huge numbers of extra computers to back the attacks, something it said might challenge Amazon.com during the busiest time of the year.

But Be'ery stressed that the boasts were unconfirmed, and the Anonymous statement said its members did not want to alienate the public by causing havoc over the holidays.

Dutch police said Friday that 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. Dutch media reported that Anonymous tried to take down the two police sites in an apparent revenge attack. Both sites were only sporadically reachable Friday morning.