Hackers today claimed to have crashed the MasterCard website in revenge for the firm suspending services to whistle-blowing website WikiLeaks.
Anonymous, understood to be a loose-knit group of internet activists, tweeted: "We are glad to tell you that www.mastercard.com is down and it's confirmed."
Another message read: "There are some things WikiLeaks can't do. For everything else, there's Operation Payback."
Mastercard was not immediately available to comment but repeated attempts to load the site met without success.
So-called distributed denial-of-service (DDoS) attacks also appeared to have been launched against PayPal, PostFinance, and the Swedish prosecutors office.
"We can confirm that there was an attempted DDoS attack on paypal.com," a spokeswoman said.
"The attack slowed some payments down for a short while but we remained fully operational throughout."
DDoS attacks, which are illegal in the UK, involve overloading a website with requests so it stops working.
"While we don't have much of an affiliation with WikiLeaks, we fight for the same reasons," the Anonymous group said in a statement on its website.
"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."
The WikiLeaks website has itself been hampered by repeated denial of service attacks and the withdrawal of services from banks and websites.
WikiLeaks relies on online donations from a worldwide network of supporters to fund its work but Visa and MasterCard yesterday suspended all payments to the whistle-blowing site.
On Monday, the Swiss post office's bank, PostFinance, shut accounts opened by WikiLeaks founder Julian Assange containing a defence fund and personal cash of 31,000 euro (26,000).
Spokesman Alex Josty said the bank's website buckled under a barrage of traffic yesterday but the onslaught seemed to have eased off.
"Yesterday it was very, very difficult, then things improved overnight. But it's still not entirely back to normal," he said.
The website for Swedish lawyer Claes Borgstrom, who represents the two women at the centre of Assange's sex crimes case, was also unreachable today.
On Saturday it emerged online payments processor PayPal had cut access for donations to WikiLeaks, with the company saying its payment service cannot be used for activities "that encourage, promote, facilitate or instruct others to engage in illegal activity".
The company providing WikiLeaks with its domain name, EveryDNS.net, also cut off service because the domain wikileaks.org was repeatedly attacked.
WikiLeaks staff complained of a series of denial of service attacks, in which thousands of computers request information at the same time.
Online store Amazon stopped hosting the site last week saying WikiLeaks did not own or control the rights to the classified content it was publishing.
WikiLeaks has said it has lost assets worth 100,000 euro (84,000) in a week as a result of the moves to end agreements with PayPal and other companies.
Founder Julian Assange was refused bail yesterday by a London court pending an extradition case over alleged sexual assaults in Sweden.