Several websites running pro-Tibet campaigns have been targeted by internet criminals, it has been claimed.

Experts at ScanSafe, an internet security firm, said that two popular websites - SaveTibet.org and FreeTibet.org - have been specifically targeted by hackers.

It is not clear who is behind the attacks, or what their motivation is, but the cyberstrikes are believed to emanate from computer servers in Taiwan and used a well-known vulnerability in some websites to link to invisible pages. These then attempt to force computers with inadequate protection to download spying programs, which can be used to track their habits or take control of their machines.

"Our initial analysis suggests that it downloads monitoring software to your computer, which would allow the attack to have remote control of those PCs," said Eldar Turvey, the head of ScanSafe. "We've contacted the sites affected and tried to give them an early warning."

The affected sites have rocketed in popularity following protests against the Chinese government in Lhasa, and at various stages of the Olympic torch relay.

A FreeTibet.org spokeswoman said campaigners were not surprised to have been attacked: "We've been targeted in many other ways in the past, and I'm actually surprised it's taken this long."

She said the site had fixed the problem after being notified of it and that no users' computers are thought to have been infected with the malicious software.