Cyber attacks apparently organised by Internet activists sympathetic to WikiLeaks founder Julian Assange shut down the website of credit card company Mastercard and two Swedish sites on Wednesday.

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"WE ARE GLAD TO TELL YOU THAT http://www.mastercard.com/ is DOWN AND IT'S CONFIRMED!," said an entry on the Twitter feed of a group calling itself AnonOps.

The move was in apparent retaliation for the company's blocking of donations to the WikiLeaks website.

Assange has angered the United States with releases of thousands of secret diplomatic cables. He is in British custody after Sweden asked him to be detained over allegations of sexual crimes were made against him by two women there.

Mastercard said in a statement it was "experiencing heavy traffic on its external corporate website - MasterCard.com - but this remains accessible...There is no impact whatsoever on MasterCard or Maestro cardholders' ability to use their cards for secure transactions."

The AnonOps group says it fights against censorship and "copywrong" and has a website entitled "Operation:Payback" at http://anonops.net/anonops/Main_Page .

Mikko Hypponen, chief research officer at security software maker F-Secure, said the typical attack was by bombarding a website with so many demands that it crashed.

The same Twitter feed said in an entry made about 17 hours ago that the Swedish prosection authority website, www.aklgare.se , was also a target.

The prosecution authority, whose arrest order led a British court on Tuesday to remand Assange in custody, said it had made a complaint to the police after an "overload attack".

"Of course, it's easy to think it has a connection with WikiLeaks but we can't confirm that," prosecution authority web editor Fredrik Berg told Reuters Televsision.

The prosecution authority website was down for most of Tuesday evening and some of Wednesday morning.

The other Swedish website targeted was that of the legal form of the lawyer for the two women who made the complaint against Assange, www.advbyra.se , which was also inaccessible.

"Well I don't know if there is a connection between our website being hacked and WikiLeaks but I suppose so," Claes Borgstrom, the lawyer for the two women, told reporters.