People in Britain, the United States and elsewhere appear to be having difficulty accessing Twitter, a day before the 2012 Olympic Games are due to officially launch and spike activity on the site.
The social network's main website is unreachable from New York, London and Johannesburg.
Twitter spokeswoman Rachel Bremmer said Thursday that "we're aware of the issue and are looking into it." She didn't immediately elaborate on the nature of the problem.
The Olympics are expected to bring an unprecedented surge of activity by sports fans on social networking sites.
At the recent European Championship final, users fired off more than 15,000 tweets-per-second, setting a sports-related record for the site.
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