An anonymous group of internet activists appear to have launched a series of cyber attacks to shut down the websites of Mastercard, a Swiss bank and the Swedish prosecutor's office in an apparent retaliation for action taken against WikiLeaks.
A group calling itself "Anonymous" or "Anonymous Operations" said on its homepage on Wednesday that Mastercard.com was its "current target", after the credit card company stopped its payment services to the whisteblowing website.
The credit card company's website was inaccessible on Wednesday morning, while PostFinance, the Swiss post office banking service, also said it was suffering denial of service attacks since it closed the bank account of Julian Assange, the website's founder.
"Since the closure of the account, groups have launched 'Operation Payback' with the aim of blocking PostFinance by simulating hundreds of thousands of connections with the aim of overloading it," Alex Josty, a PostFinance spokesman said.
'Operation avenge Assange'
Hackers appeared to be using denial service attacks against the websites, in which computers across the internet are harnessed to jam sites with an overload of requests for data, stopping their sites from functioning.
One tweet on Anon_Operations said: "Target: postfinance.ch : Grab your weapon and its settings: FIRE NOW!".
Viral messages have also been posted on the internet, some called "Operation Avenge Assange", urging people to campaign on behalf of WikiLeaks and take part in "the first info war ever fought".
The call comes after John Perry Barlow, co-founder of the San Francisco-based internet freedoms group Electronic Frontier Foundation, tweeted: "The first serious infowar is now engaged. The field of battle is WikiLeaks. You are the troops".
"Well I don't know if there is a connection between our website being hacked and WikiLeaks but I suppose so"
Claes Borgstrom, Swedish lawyer
In a statement on its website, the group Anonymous, which is a leaderless group of activists campaigning for online freedoms, said: "While we don't have much of an affiliation with WikiLeaks, we fight for the same reasons".
"We want transparency and we counter censorship ...This is why we intend to utilise our resources to raise awareness, attack those against and support those who are helping lead our world to freedom and democracy."
Attack on Swedish prosecution
The website of the Swedish prosecution authority, which has brought an arrest order against Assange, who is currently being held at a prison in Britain, said it had also made a complaint to police after an "overload attack" on Tuesday evening.
Another Swedish website for the the lawyer of the two women who made the complaint against Assange was also inaccessible.
"Well I don't know if there is a connection between our website being hacked and WikiLeaks but I suppose so," Claes Borgstrom, the lawyer for the two women, said.
The attacks against the websites come after a number of companies, including Visa Europe, Mastercard, PayPal and Amazon cut off ties with WikiLeaks after the organisation angered the US by releasing thousands of secret diplomatic cables.
News website CNET cited a MasterCard spokesman as saying the whistleblowing website was being cut off due to rules barring use for "directly or indirectly engaging in or facilitating any action that is illegal."
WikiLeaks has also found it more challenging to host its information online, with the website coming under an increasing number of cyber attacks leading one of its US server providers to end its relationship with the organisation.