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 over 40,000, with thousands of new downloads reported overnight.
"It's definitely increasing," Imperva Web researcher Tal Be'ery said in a telephone interview from Israel.
The freely available software is a critical part of the campaign by "hacktivists" seeking to take revenge on sites they believe have betrayed WikiLeaks, the group that 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 generally been successful in foiling their enemies. 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 part of its website inoperative. Moneybookers.com, another targeted site, was inaccessible Friday.
WikiLeaks has been careful to distance itself from Anonymous, saying "we neither condemn nor applaud these attacks."
A press release circulated under the Anonymous name Friday said the group was acting "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 in huge numbers of extra computers to back the attacks _ something it said might challenge Amazon.com _ another site that cut its ties to WikiLeaks _ at one of the retailer's busiest times of the year.
But 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.
"Simply put, attacking a major online retailer when people are buying presents for their loved ones would be in bad taste," the Anonymous release said.
Dutch police said Friday 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 and alleged WikiLeaks supporter.
In Australia, WikiLeaks supporters held rallies Friday in Brisbane and in Sydney, where more than 500 people gathered outside Town Hall, some waving signs that read, "Hands off WikiLeaks, We deserve the truth," and "Don't shoot the messenger."
One man sealed his mouth shut with tape on which the words "NO LEAKS" was written.
Among the most recent newsworthy WikiLeaks revelations was a claim that drug maker Pfizer Inc. hired investigators to dig up dirt on Nigeria's former attorney general in a bid to stop action over a 1996 drug study, and that the U.S. considered taking military action against an arms-laden Ukrainian ship after it was hijacked by Somali pirates two years ago.
The U.S. Department of Justice, meanwhile is considering whether to charge those behind the leaks under the espionage act or other laws, while U.S. diplomats, deeply embarrassed by WikiLeaks' disclosures, have struggled to contain the fallout.
"The deplorable WikiLeaks disclosures put innocent lives at risk, and damage U.S. national security interests," U.S. Ambassador to London Louis Susman wrote in an editorial Friday in The Guardian newspaper. "There is nothing brave about sabotaging the peaceful relations between nations on which our common security depends."
The U.S. may soon be facing more than WikiLeaks as an opponent.
A former WikiLeaks spokesman plans to launch a rival website Monday called Openleaks that will help anonymous sources deliver sensitive material to public attention. Daniel Domscheit-Berg made the claim in a documentary by Swedish broadcaster SVT airing Sunday but obtained in advance by the AP.
WikiLeaks founder Julian Assange remained in a U.K. jail ahead of a Dec. 14 hearing where he plans to fight Sweden's request to extradite him to face sex crimes allegations.
Kristen Gelineau in Sydney, Michael Corder in The Hague and Louise Nordstrom in Stockholm contributed to this report.