WikiLeaks activists instructed their followers on Thursday to mount a distributed denial of service attack on a PayPal website that manages the integration of the company's payment processing technology with independent online merchant websites. Paypal is a subsidiary of eBay.

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A PayPal spokesman said the company had detected an attack on the site, http://api.paypal.com, but that it appeared to be operational, although various attempts to access the website by Reuters on Thursday were unsuccessful.

Online retail and web-hosting powerhouse Amazon last week stopped hosting WikiLeaks' website, and on Thursday it briefly became the main target of the pro-WikiLeaks campaigners - before they admitted it was too big for them, for the moment.

"We cannot attack Amazon, currently. The previous schedule was to do so, but we don't have enough forces," read one message on Twitter.

The activists said they would instead attack PayPal, which has suspended the WikiLeaks account the organization had used to collect donations. MasterCard and Visa had also become targets after stopping processing donations.

By early evening EST (1810 GMT), the main websites of PayPal, Amazon - a key Christmas shopping destination - MasterCard and Visa all appeared to be functioning normally.

Operation payback

Facebook said it had removed the activists' Operation Payback page on Thursday because it was promoting a distributed denial of service attack - a form of freezing websites by bombarding them with requests that is illegal in many countries.

The campaign also disappeared briefly from Twitter before reappearing in a different guise. Twitter declined to comment.

In an online letter, Anonymous, a loose-knit group, said its activists were neither vigilantes nor terrorists. It added: "The goal is simple: Win the right to keep the Internet free of any control from any entity, corporation, or government."