Visitors to the social networking site early in the morning were greeted with the familiar "fail whale" on their computer screens, instead of a functioning site. The cartoon whale is Twitter's way of trying to lessen user frustration during such incidents.
Twitter at first said the outage was "due to an extremely high number of whales." The site got more serious after it had recovered at about 8 a.m. Eastern time.
"A sudden failure coupled with problems in switching to a backup system produced a high number of errors for around 90 minutes," the company said on its blog. "This made the site largely inaccessible."
Twitter said no data was lost in the outage and that the site had not suffered a security breach.
The microblog service, founded in May 2007, has suffered technical problems in the past, which have been attributed to its rapid growth. The Web site enables people to broadcast messages of not more than 140 characters to networks of people.
Twitter will slow site performance by disabling features, such as custom searches, in order to accommodate heavy traffic, according to Rich Miller, editor and founder of Data Center Knowledge, which provides analysis of the data center industry.
"In recent weeks Twitter.com users have periodically encountered messages that the service was over capacity, but the condition was usually temporary," Miller wrote on his blog.
The latest outage was the longest sustained downtime since a denial-of-service attack in August that also affected other social networking sites. The attack brought the site down for more than two hours and slowed access to Blogger, Facebook and LiveJournal.