Hackers acting in support of WikiLeaks and its founder Julian Assange attacked the online services of Visa and MasterCard Wednesday, and warned of further disruptions at social-networking site Twitter.

The group, operating under the name "Anonymous," said it is targeting companies that have severed ties with WikiLeaks in the wake of the whistle-blowing website's ongoing drive to publish previously secret U.S. diplomatic cables.

The so-called "hacktivists" hit the global MasterCard website, after that company stopped processing online donations to WikiLeaks. Later Wednesday, the website of Visa, which also cut off WikiLeaks from its services, was inaccessible.

The group, which operates as part of "Operation Payback," which has previously targeted the Church of Scientology and the music industry, did not claim responsibility for all of the attacks. But in a Twitter message on Wednesday, they claimed responsibility for the attacks on MasterCard and Visa. MasterCard acknowledged a "service disruption" of its Secure Code system for verifying online payments, but a spokesperson said customers were still able to use their credit cards securely.

The hackers were using so-called denial of service attacks -- events that overwhelm websites with requests for data -- to hit their targets. PayPal, which cut ties to WikiLeaks over the weekend, said it was business as usual on Wednesday, but confirmed it was the victim of a "dedicated denial-of-service attack" on Monday.

While Wednesday's attacks were far-reaching, there may yet be more. "Operation Payback" said in a statement late Wednesday that Twitter will be in its crosshairs next, claiming that the social media site is "censoring Wikileaks discussion."

Some Twitter users say the site is blocking the word "WikiLeaks" from appearing on its list of trending topics. However, the company denies any interference in the trending topics listing, saying it is determined by an algorithm.

On their website, the hackers said that "while we don't have much of an affiliation with WikiLeaks, we fight for the same reasons."

The group also said that it is pro-transparency and against censorship and it intends to "attack those against and support those who are helping lead our world to freedom and democracy."

While the group claimed honourable intentions, it clearly made enemies on Wednesday. Late in the day, many sites affiliated with "Operation Payback" were disrupted by unknown counter-hackers.

CTV tech expert Kris Abel said Wednesday consumers should not be concerned that their own credit-card accounts or personal information were put in danger by the attacks.

"They're not attacking MasterCard themselves, they're not breaking in trying to gain access to money or to accounts or to passwords, but to simply try to refuse the website from being able to access the Internet and for its users to be able to access it in order to draw attention to its cause," Abel told CTV's Power Play.

Abel said the attacks are relatively harmless and brief, and said it is unlikely the hackers could sustain a long-term shut-down in service, which is likely not their goal anyway.

"This is kind of a new form of behaviour that we're seeing," Abel said. "The idea of hacktivism, of getting online and trying to perpetuate an attack for an ideal goal, not specifically to try to gain in profit or to gain in resources, but simply to try to enter a political arena."

The hackers also targeted Swedish prosecutors and a Swiss financial group tied to Assange.

Since WikiLeaks began publishing hundreds of thousands of diplomatic cables and other documents, it became publicly known that Assange is wanted for questioning on sex-crimes allegations in Sweden, which pertain to sexual encounters with two separate women in that country.

Assange says the charges are bogus and he surrendered to British authorities Tuesday, so that he could begin fighting extradition proceedings that seek to bring him to Sweden. He is currently in custody in London.

On Wednesday, the websites belonging to Swedish prosecutors and to the lawyer representing the two women who made accusations against Assange came under attack by the hackers.

A Swiss authority that froze Assange's bank account also came under attack on Wednesday.

"Yesterday it was very, very difficult, then things improved overnight," Postfinance spokesperson Alex Josty said Wednesday. "But it's still not entirely back to normal."

As many leaders have criticized WikiLeaks for publishing U.S. government documents about the wars in Iraq and Afghanistan, there are some who say Washington should take a share of the blame.

Kevin Rudd, the Australian foreign minister and the country's former prime minister, said "the core of all this lies with the failure of the government of the United States to properly protect its own diplomatic communications."

Citing the hundreds of thousands of Americans who had access to the documents before they were leaked, Rudd said "that's where the problem lies."

With files from The Associated Press