Some of the motivation for the cyber campaign appears to stem from anger at the arrest in Britain of Assange over alleged sex crimes committed in Sweden. He is in jail in London, awaiting an extradition hearing.

Don't miss

Assange said last week he had expected clampdowns in countries such as the United States that championed free speech, and had deliberately picked providers like Amazon to host its data to test that theory.

UN High Commissioner for Human Rights Navi Pillay voiced concern on Thursday at reports of pressure being exerted on private companies to halt financial or Internet services for WikiLeaks.

"The campaign is not over from what I've seen, it's still going strong. More people are joining," a spokesman for the Anonymous group calling himself "Coldblood" told BBC Radio 4. The speaker, who had an English accent, said he was aged 22 and was a software engineer.

"Anonymous has targeted mainly companies which have decided for whatever reason not to deal with WikiLeaks. Some of the main targets involve Amazon, MasterCard, Visa and PayPal."

Limited interruption

In a statement on Thursday, MasterCard said although there was a limited interruption of some online services, cardholders could continue using cards for transactions worldwide. Its main processing systems were not compromised, the statement said.

The campaigners also claimed responsibility for bringing down Visa Inc's site, which was temporarily unavailable in the United States, but later restored. Swedish newspaper Aftonbladet said the Swedish government's website was down for a short time overnight in the latest apparent attack.

Assange, a 39-year-old Australian, has been hailed as an advocate of free speech by supporters, but now finds himself fighting serious sexual allegations made by two women in Sweden.

Assange will have another court appearance next Tuesday and his supporters assert he is being victimized for his work.