An online group called Anoymous used micro-blogging service Twitter to claim responsibility for the distributed denial of service (DDOS) attach.
"We are glad to tell you that http://www.mastercard.com/ is down and it's confirmed! #ddos #wikileaks Operation:Payback(is a bitch!) #payback" read the tweet from Anon_Operation.
MasterCard revealed earlier this week that it would not process any more donations to the website, which recently published more than 250,000 sensitive cables sent among members of the US State Department, as it's concerned that the website is taking part in illegal activity.
"MasterCard rules prohibit customers from directly or indirectly engaging in or facilitating any action that is illegal," said Chris Monteiro from the credit card company.
The Anoymous group is thought to be linked to online forum 4Chan, which launched a DDOS attack on ACS:Law, a legal firm that has been chasing illegal downloaders, in September.
Mastercard isn't the only company freezing payments to WikiLeaks. Swiss bank PostFinance, which has already been attacked by the group, and PayPal are among other copmpanies boycotting WikiLeaks donations. Anoymous has threatened to target PayPal next.
Datacell, the firm that enables WikiLeaks to accept donations via Mastercard, has vowed to take legal action to ensure payments can be accepted once again, as it believes freezing payments could subsequently harm Datacell's business.
WikiLeaks chief spokesman Julian Assange was arrested in the UK yesterday. He is wanted in Sweden on one count of unlawful coercion, two counts of sexual molestation and one count of rape.