The websites of Visa and Mastercard were sabotaged Wednesday in what is believed to have been an act of retribution by supporters of WikiLeaks against companies that have cut off the whistleblower site.

The U.S. site of Visa was still inaccessible as of late Wednesday afternoon. but both the U.S. and Canadian sites of MasterCard were back up after being down most of the day, the victims of a cyberattack dubbed Operation Payback.

Visa and MasterCard suspended WikiLeaks's accounts with them on Tuesday, effectively cutting off donations to the site made using those credit cards. Internet payment company PayPal has also shut down WikiLeaks's account, as has Amazon's web hosting arm, whose servers WikiLeaks had been using, and the DNS provider EveryDNS.

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Supporters of WikiLeaks founder Julian Assange gather outside the City of Westminster Magistrates' Court in London where his case was heard. WikiLeaks supporters are also fighting back online. ((Lefteris Pitarakis/Associated Press))

On Twitter, a group of activists using the catch-all moniker Anonymous took credit for shutting down Visa.com Wednesday, writing: "IT’S DOWN! KEEP FIRING!!! #DDOS #PAYBACK #WIKILEAKS."

MasterCard Canada spokesman Jim Issokson told CBC News earlier in the day the denial of service attack on the company's website had not affected any credit card transactions.

"MasterCard's systems have not been compromised," he said, "At this time, the issue appears to be the result of a concentrated effort to flood our corporate website with traffic and slow access.

"There is no impact on our cardholders' ability to use their cards for secure transactions globally."

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The sabotage took the form of attacks in which computers across the internet are harnessed — sometimes surreptitiously — to jam target sites with mountains of requests for data, knocking them out of commission.

Early Wednesday, the activists operating as Anonymous, often used as a stand-in term for various sites and internet users acting in consort, led media to a YouTube video in which they stated the objectives of Operation Payback.

Wave of support

Wednesday's cyberattacks are part of a wave of support for WikiLeaks that is sweeping the internet. Twitter was choked with messages of solidarity Wednesday, and the site's Facebook page hit one million fans.

More than 1,200 mirror sites had been set up by Wednesday afternoon to host WikiLeaks content that was no longer available at WikiLeaks.com, which is now operating at the IP address 213.251.145.96.

Anonymous has promised to come to Assange's aid by knocking offline websites seen as hostile to WikiLeaks.

'We want transparency, and we counter censorship.' — Message from internet activists supportinG WikiLeaks

"While we don't have much of an affiliation with WikiLeaks, we fight for the same reasons," the group said in a statement. "We want transparency, and we counter censorship .…This is why we intend to utilize our resources to raise awareness, attack those against, and support those who are helping lead our world to freedom and democracy."

Hackers have also attacked the websites of the Swedish prosecutors pursuing charges of sexual assault against WikiLeaks co-founder Julian Assange.

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Claes Borgstrom, lawyer for the two women who claim to have been sexually assaulted by WikiLeaks founder Julian Assange in Sweden in August, talks to media at his office in Stockholm Wednesday. ((Anders Wiklund/Associated Press))

The website of Swedish lawyer Claes Borgstrom, who represents the two women at the centre of Assange's sex crimes case, was unreachable Wednesday.

The Swiss postal system's financial arm, Postfinance, which shut down Assange's bank account on Monday, was also having trouble with its site. Spokesman Alex Josty said the website buckled under a barrage of traffic Tuesday, but the onslaught seems to have eased off.

"Yesterday, it was very, very difficult; then, things improved overnight," he told The Associated Press Wednesday. "But it's still not entirely back to normal."

While one internet company after another has cut its ties to the websites amid intense U.S. government pressure, the French government's effort to stop a company there from hosting WikiLeaks has failed — at least for now.

The web services company OVH, which is among those hosting the WikiLeaks.ch site, sought a ruling by two courts about the legality of hosting WikiLeaks in France. The judges said this week they couldn't decide on the highly technical case right away.

WikiLeaks evoked the ire of the U.S. government last spring when it posted a gritty video taken by army helicopters showing U.S. troops in Iraq gunning down two unarmed Reuters journalists. Since then, the organization has leaked some 400,000 classified U.S. war files from Iraq and 76,000 from Afghanistan that U.S. military officials say included names of U.S. informants and other information that could put people's lives at risk.

The latest leaks have involved private U.S. diplomatic cables that included frank U.S. assessments of foreign nations and their leaders.

With files from The Associated Press