Internet hacktivists are claiming to have brought down the Mastercard website as revenge for the firm withdrawing services to Wikileaks.
The Anonymous group of hackers have also brought down the website of the Swedish prosecutors office which is pursuing founder Julian Assange.
It has pledged to launch denial-of-service attacks on websites it sees as anti-Wikileaks.
Earlier it hit the Swiss bank that froze Mr Assange's assets.
PayPal, which has stopped processing donations to Wikileaks, has also been targeted.
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.
Mastercard said that it had nothing to say about the matter at the time of publication.
But 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, experts say. It is not clear whether the attacks have affected Mastercard's payment system.
Earlier the group 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," the group said in a statement.
Noa Bar Yosef, a senior analyst at security firm Imperva said the attacks are "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.
Before the Mastercard attack, a member of Anonymous, who calls himself Coldblood, told the BBC that "multiple things are being done".
"Websites that are bowing down to government pressure have become targets," he said.
"As an organisation we have always taken a strong stance on censorship and freedom of expression on the internet and come out against those who seek to destroy it by any means."
"We feel that Wikileaks has become more than just about leaking of documents, it has become a war ground, the people vs. the government," he said.
Some of the early DDoS hits failed to take sites offline, although that was not the point of the attacks, according to Coldblood.
"The idea is not to wipe them off but to give the companies a wake-up call," he said. "Companies will notice the increase in traffic and an increase in traffic means increase in costs associated with running a website."
DDoS attacks are illegal in many countries, including the UK.
Coldblood admitted that such attacks "may hurt people trying to get to these sites" but said it was "the only effective way to tell these companies that us, the people, are displeased".
Anonymous is also helping to create hundreds of mirror sites for Wikileaks, after its US domain name provider withdrew its services.
The attacks are part of an ongoing infowar involving Wikileaks.
The whistle-blowing site has also been hit by a series of DDoS attacks, following the release of a quarter of a million US embassy cables.
It is unclear who is behind the attacks but it seems that Wikileaks is getting too hot to handle as many of the businesses that work with the site, distance themselves from it.
On 3 December, domain name provider EveryDNS cut off service, citing the denial-of-service attacks as the reason.
Amazon also ended an agreement to host the site, saying Wikileaks failed to adhere to its terms of service.
It said that Wikileaks was unable to ensure that it "wasn't putting innocent people in jeopardy" by leaking classified documents.
Online payment company, PayPal, has permanently restricted Wikileaks' account, making it harder for supporters to make donations.
MasterCard Worldwide is also choking payments to the site.
The Swiss bank, PostFinance has closed the account of Wikileaks founder Julian Assange.
In all cases, the companies have insisted their decisions are not politically motivated.
PayPal said Wikileaks' account had violated its terms of services.
PostFinance, meanwhile, claimed Assange had provided false information when opening his account.
But some have taken a different view.
French internet service provider OVH said it had no plans to end the service it provides to Wikileaks.
"OVH is neither for nor against this site. We neither asked to host this site nor not to host it. Now it's with us, we will fulfil the contract," said OVH managing director Octave Klaba.
"It's neither for the political world nor for OVH to call for or to decide on a site's closure," he added.
French industry minister Eric Besson had called for the site to be shut down, saying France could not host internet sites that "violate the confidentiality of diplomatic relations and put in danger people protected by diplomatic secrecy".
But on 6 December, a French judge declined to force OVH to shut Wikileaks down, saying the case needed further argument.
Wikileaks has amassed some high-profile enemies including Senator Joe Lieberman, who chairs the US Homeland Security Committee.
He has urged the US government to "use all legal means necessary to shut down Wikileaks before it can do more damage by releasing additional cables".
Dr Joss Wright, a research fellow at the Oxford Internet Institute thinks it could be too late to legislate Wikileaks offline.
"Wikileaks has released an encrypted file containing all of the embassy cables," says Dr Wright. "The information is already out there."
Dozens of copies of that encrypted file have been shared using peer-to-peer networks, such as BitTorrent. "Once the information is there, it's virtually impossible to stop people sharing it," said Dr Wright.
Founder of Wikileaks Julian Assange has been arrested in London and is being held in custody, having been refused bail.
He is accused by the Swedish authorities of sexual assault.
If anyone needs some more "insider" information, I'm in the IRC server where this is taking place, and have been for like...a month, haha. I'm not DDoSing, but watching. This originally started as Operation Payback wehre they hit anti-piracy websites (including genesimmons.com) but now they're supporting Wikileaks. I have never seen an Anonymous DDoS like this, not even during Operation Titstorm which had hundreds of people involved. At the time of me writing this, there are over 1300 people connected to the IRC channel orchestrating this, and close to 850 people in "Hivemind" which is when you hook up LOIC (low orbit ion cannon) to an IRC channel, and the operator of the IRC channel gives commands about who to attack, and your computer then does so. In other words, a voluntary botnet. Also, I know one guy who says he has 2500 strong botnet, and provided evidence. It's 4chan though, so whether he's lying or telling the truth...wouldn't be a surprise either way, haha.
The site hit yesterday was postfinance.ch which froze Assange's account. For almost an entire day they prevented people from making transfers on that site. Postfinance is kinda like a half-post-office, half-bank sorta *thing*. Anyway, it's a large institution, in the top 10K sites online, and has a stadium named after it. For a short period they also attacked lieberman.senate.gov, which actually semi-worked. The server was working about 50% of the time. When you consider the fact that liberman.senate.gov has the same IP as senate.gov...that's pretty impressive.
EDIT: owner of IRC channel says that when hivemind his 1000 they're going to go after paypal again. If that happens...jesus christ, help us all.