According to overnight news report, the Anon_Operation said in one Twitter message that "www.mastercard.com/ is down" and the site of the credit card giant could not be accessed from Geneva.
Infosecurity's attempts to access the site via the MANAP and London Telehouse peering points – the two UK primary IP peering points in the UK – timed out, although one single session (out of ten) succeeded through a small proxy service in Vienna, VA, in the US.
Reports are also coming in that PostFinance, the Swiss post office banking service, which closed Assange's bank account earlier in the week, has also seen its website effectively downed by a DDoS attack.
In a press statement, PostFinance said: "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."
The Agence France Press newswire says that the Anon Operation tweeted a message to say that: "Target: postfinance.ch :: Grab your weapon and its settings: FIRE NOW!" said one tweet on Anon_Operation."
PostFinance is reported as saying it closed a bank account set up by Assange because he gave "false information" on his address.
IT security industry reaction to the DDoS attacks suggests that many observers were expecting this sort of action.
Alan Bentley, international senior vice president of Lumension, said that the attack on MasterCard's website following its move to block payments to WikiLeaks shouldn't surprise anyone.
"WikiLeaks has a strong following amongst the hacker community and it was inevitable that there was going to be some form of backlash", he said, adding that many disgruntled hacktivists will be seeking revenge on behalf of the whistleblowing website.
"It is [also] highly likely that this will be the first in a series of attacks on businesses such as Amazon, PayPal, Visa and Swiss Bank, all of which withdrew services to WikiLeaks over recent days", he added.
According to Bentley, the WikiLeaks saga is undoubtedly set to continue for some time, and all organisations involved will want to beef up their security efforts in a bid to protect themselves from the wrath of the hactivist community.