For Twitter's more than 30 million users, life — even a few hours — without the popular service meant no tweeting about news, work or the fact that Twitter was down.

Online networks Twitter, Facebook and LiveJournal yesterday morning were overwhelmed by denial-of-service attacks disrupting access to about 300 million users. Botnets — thousands of infected home and workplace PCs — flooded the Web sites with nuisance requests, thus cutting off access to anyone else.

Anand Chandrasekaran, a serial entrepreneur in Mountain View, Calif., said he felt "completely cut off" from hundreds of friends without Twitter. He used Facebook and e-mail instead.

Security experts can't say if the attacks were related. Twitter users around the globe could not tweet for at least three hours. Access was restored in much of the U.S. by 1 p.m. EDT, but Twitter could not be reached via iPhone or in Eastern Europe through much of the day, said Stefan Tanase, a senior analyst at Kaspersky Lab.

"This was definitely a pretty heavy attack," Tanase said.

Passionate Twitter users bang out 140-character updates — or tweets — on what they're thinking, and share news and ideas. Companies such as Comcast and JetBlue use tweets to provide customer service. And celebrities use it as a marketing tool.

Facebook reported degraded service for some of its 250 million users, while LiveJournals said its 21 million users were cut off for an hour.

Nothing on this scale has been seen since February 2000, when a Montreal teen, known as Mafiaboy, directed a bot network to cut off access to Yahoo, eBay, Amazon and other sites.