BBC.Micro-blogging service Twitter was taken offline in what the company believes was a co-ordinated attack.
The popular site has been subject to a so-called denial-of-service attack, according to the firm's official blog.
Denial-of-service attacks take various forms but often involve a company's servers being flooded with data in an effort to disable them.
The outage began Thursday morning but the geographic scale of the attack is still not known.
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