Millions of Twitter users were blocked from their feeds on Tuesday as the social networking site went down in several countries, suffering one of the worst outages in its 10-year history.
"Some users are currently experiencing problems accessing Twitter. We are aware of the issue and are working towards a resolution," the US site said, without giving details of the cause of the outage.
A spokesperson for Twitter Europe confirmed the site was down, while users in South Africa, Brazil, the Philippines, Nigeria and Uganda also reported problems accessing the site with the hashtag #twitterdown on Facebook.
Users in affected countries were met with the message: "Something is technically wrong."
After a shutdown lasting about an hour on Tuesday morning, the site was back online in London and Paris - but users in both cities experienced intermittent service in the afternoon.
Users elsewhere, including Russia, were also still experiencing problems.
Many took to Facebook to report the outage with the hashtag #twitterdown.
The shutdown was also the butt of jokes from some social media channels, with British TV channel E4 posting on its Facebook page: "Twitter is down. Worker productivity hits record levels. UK is out of recession!!!"
Rick Devens, a local US radio host, posted on Facebook: "We may never get back some of the clever thoughts I had during this tragic outage."
Twitter has regularly suffered outages since its launch in 2006, but generally of a shorter duration. Its social media rivals Facebook and Instagram have also had to deal with brief shutdowns.
During a site breakdown in July 2012, frustrated Twitter addicts were unable to post for two hours due to problems at its data centres.
Twitter shares have tumbled 20 percent on the New York stock exchange since the start of the year - notably on rumours that it may scrap the 140-character limit that has kept its posts pithy for the last decade.
Tech news website re/code reported this month that tweets of up to 10,000 characters could be available from March, winning a mixed response from netizens.
Twitter chief executive Jack Dorsey did not confirm the plan, but said the company is "not going to be shy about building more utility and power into Twitter."
The site continued to lose money over the first three quarters of last year. In October it reported a third quarter loss of $132 million, on a disappointing 11 percent year-on-year increase in regular users to 320 million.