Russian hackers are believed to have been behind a “single, massively coordinated attack” on some of the world’s leading websites last week — all in an effort to silence a pro-Georgian blogger.

Millions of people were locked out of Twitter and Facebook after cyber-attacks on the social networking sites on Thursday. LiveJournal, a blogging service, was also hit, although Google, the world’s most popular site, said that it was able to repel the onslaught.

Under such “denial-of-service” attacks, sites are deluged with traffic in an attempt to bring them down.

The blogger from Georgia — who uses the name Cyxymu and had accounts on all the sites — was revealed yesterday to be the target of the attacks. He blamed the Kremlin: “Maybe it was carried out by ordinary hackers but I’m certain the order came from the Russian Government,” he told The Guardian’s website. “An attack on such a scale that affected three worldwide services with numerous servers could only be organised by someone with huge resources.”

Cyxymu, whose real name is Georgy, is a 34-year-old lecturer and outspoken critic of the Russian Government. Yesterday Facebook confirmed that Cyxymu was the focus of the attack, “to keep his voice from being heard”.

Cyxymu’s blog was not accessible yesterday, but an archived version showed that he was aware of the cyber-strike as it was happening: “Now it’s obvious it’s a special attack against me and Georgians,” read his message in Russian.

Experts said evidence suggested that the attacks originated in the Abkhazia region, a disputed Black Sea territory between Russia and Georgia. Some said that the attack may have been in retaliation for “propaganda” on the blog.

Yesterday the two countries marked a year since the outbreak of their war, amid enduring tensions in the region. Russia recognised the breakaway territories of South Ossetia and Abkhazia as independent after the conflict.

Russia insists that it moved into Georgia to defend South Ossetia from a Georgian attack aimed at retaking the rebel territory, while Georgia claims that it faced a “large-scale Russian invasion”.

Cyber-attacks were used by both sides during the conflict.

The microblogging site Twitter was overwhelmed by the assault and could not be accessed for hours on Thursday. Facebook members experienced delays logging in and posting to their profiles.

Biz Stone, the co-founder of Twitter, said that the “continuing denial-ofservice attack is being mitigated, although there is still degraded service for some folks while we recover completely. Such attacks are malicious efforts to disrupt services.”

In a typical denial-of-service attack, the controller uses botnets — made up of thousands of virus-infected PCs — to target a site. The motive may be to blackmail owners of a site into paying the attackers to stop. However, in this case the motive appears to have been political.

Graham Cluley, of Sophos, an internet security company, said: “It’s a bit like 15 fat men trying to get through a revolving door at the same time: nothing can move.”

A Google spokeswoman said: “We are aware a handful of non-Google sites were impacted by a DoS attack this morning. Google systems prevented substantive impact to our services.”

It also emerged that hackers sent a flurry of spam e-mails that contained links to pages on Twitter, Facebook and other sites written by Cyxymu.

Bill Woodcock, research director of the San Francisco-based Packet Clearing House, a charity that tracks internet traffic, said that when people clicked the links they were taken to the activist’s web pages, but the process of loading pages at such volumes overwhelmed servers.

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