Many users were faced with an error page after logging in to Twitter
Social network Twitter is offline for many users, with web visitors being greeted by an error page.
Some mobile users are also unable to read or post tweets to the network, which has over 300 million active members.
Twitter chose to communicate the problem via a tweet, in which the firm said it was aware of the problem.
Some users took to Facebook to complain.
A spokesman for Twitter said that a tweet from the company's @support account read: "Some users are currently experiencing problems accessing Twitter. We are aware of the issue and are working towards a resolution."
Twitter users including the BBC's technology correspondent took to Facebook to highlight the fault
The issue began shortly after 0800 GMT, according to Down Detector.
For some, the site appears to be intermittently accessible while the issue persists.
"I've had to talk to real people and that's a little bit frightening," joked social media consultant David Schneider.
"Genuinely, I didn't know where to get my news from, I usually put a tweet out in the morning."
Mr Schneider, who runs consultancy That Lot told the BBC that he did feel there was a genuine sense of community on Twitter which he missed.
"I just hope we'll get through the trauma," he joked.
The fault would probably seem like an "unwelcome headache" for Twitter chief executive Jack Dorsey, according to consultants Frost & Sullivan.
There were also issues with Twitter's application program interface (API), which allows websites and apps to connect to Twitter, principal analyst Sheridan Nye told the BBC.
"The loss of the micro-blogging site's APIs multiplied the issue for all manner of developers, companies and organisations," she said.
Ms Nye added that there could be serious consequences for some businesses if downtime went on for long periods.
"Any company that was in the process of launching a new digital product would have lost valuable marketing insight," she said.
Recently launched apps might not have been able to take advantage of live feedback on the site, Ms Nye said - something which Barclays took advantage of following the launch of the bank's PingIt payments app last year.
"Barclays found out that under-18s were annoyed that they couldn't use the service - so Barclays quickly modified the service to allow them access," Ms Nye said.