Twitter stopped working at about 8 a.m. PT, a day before the 2012 Olympic Games, which were expected to cause a spike in the use of the site.
The company acknowledged the problem, only saying in a statement that its engineers are "currently working to resolve the issue." On its site, a message was posted that said, "Twitter is currently down for %(equals) reason %. We expect to be back in %(equals) deadline %."
The Twitter outage was reported from countries in North America, Europe, Asia, the Middle East, and Africa. Some users were able to post tweets through a mobile site or third-party applications.
Updates about the Olympic torch - which is currently winding its way through central London - were still pouring in, but far more slowly than earlier in the day.
Just after 10 a.m. PT, Twitter seemed to have resolved the issue.
In June, Twitter blamed its outage on a "cascaded bug" in one of its infrastructure components.
ABC News and The Associated Press contributed to this report.