What might have been no more than a teenage prank knocked Twitter, the fast-growing internet communications service, offline for more than two hours yesterday.

The micro-blogging firm, whose service allows text and web posting of messages of 140 characters or less, said it was hit by a denial-of-service attack, in which thousands of personal computers attempt simultaneous connections, slowing the target site's response to a virtual standstill.

The same type of cyber-attack crippled US and South Korean government websites last month, although Twitter has fewer obvious enemies. Other victims of denial-of-service attacks yesterday included the blogging company LiveJournal, which was shut down for a few hours and the social network Facebook, which slowed to a crawl. The popular Gawker network of news and gossip sites was shut down by a similar attack on Monday.