Internet hacktivists have claimed they brought down the Mastercard website.
The Anonymous group of hackers have also brought down the website of the Swedish prosecutors office which is pursuing founder Julian Assange.
But Mastercard said there was "no impact" on people's ability to use their cards for transactions.
Anonymous has claimed to have hit several targets including the Swiss bank that closed Wikileaks' head Julian Assange's account.
PayPal, which has stopped processing donations to Wikileaks, has also been targeted.
The online payment firm has admitted that it stopped payments following a request from the US government.
"State Department told us these were illegal activities. It was straightforward," PayPal's Osama Bedier told the Le Web conference in France.
PayPal had originally said Wikileaks' account had violated its terms of services.
Swamp site
Anonymous is a loose-knit group of hacktivists, with links to the notorious message board 4chan.
"We are glad to tell you that Mastercard is down and it's confirmed," the group tweeted on 7 December.
Soon after, Mastercard said the site was still functioning.
"Mastercard is experiencing heavy traffic on its external corporate website - Mastercard.com - but this remains accessible," said Doyel Maitra of the firm.
"We are working to restore normal speed of service. There is no impact whatsoever on Mastercard or Maestro cardholders' ability to use their cards for secure transactions."
Security experts have said the site has been under a so-called distributed denial-of-service attack (DDoS), which swamp a site with so many page requests that it becomes overwhelmed and drops offline.
Access to the website appears to be possible intermittently and it is still visible from some countries.
Noa Bar Yosef, a senior analyst at security firm Imperva said the attacks were "very focused".
"It is recruiting people from within their own network. They are actually asking supporters to download a piece of code, the DDoSing malware, and upon a wake-up call the computer engages in the denial of service," he said.
'Wake-up call'
Earlier Anonymous confirmed other targets: "In response to the arrest of Julian Assange, Anonymous has taken down PostFinance.ch, who terminated Wikileaks bank account, using a distributed denial-of-service attack.
"Subsequently, Anonymous attacked
http://www.aklagare.se, the Swedish Prosecutors office, also using a DDoS attack, and took the site down in under 10 seconds of beginning the attack."
Before the Mastercard attack, a member of Anonymous, who calls himself Coldblood, told the BBC that "multiple things" were being done to target companies that had stopped working with Wikileaks or which were perceived to have attacked the site.
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