In a status update at 1800 GMT (1 p.m. EST), Twitter said an "intermittent issue affecting some users" was related to "an internal code change."
"We reverted the change, which fixed the issue," Twitter said in a statement. There was no immediate way to determine whether full service had been restored for all users.
Twitter Inc shares were down almost 7 percent at market close. Wall Street has long worried about Twitter's stagnant growth in users and advertising revenue, and analysts said the outage added to the concern.
"The current market malaise and the recent site outages are compounding the negatives and having a very negative reaction on the shares," said Victor Anthony, Axiom Capital Management analyst.
Some users who tweeted with the hashtag #twitterdown reported they had not experienced problems or that their service had been restored. Others said they were still having problems after Twitter's announcement.
Many pointed out that Twitter could not have been down for everyone since #twitterdown was among the top trending hashtags on the site.
Both Twitter's Internet and mobile services began experiencing outages concentrated in northern Europe around 0820 GMT.
Users from Scandinavia to Saudi Arabia to South Africa reported outages. India and Russia also suffered performance issues, according to a Twitter technical site.
Intermittent breakdowns later spread to the United States and Canada in the early part of their working day.
Sporadic disruptions continued at 1420 GMT, six hours after they first began to spread. At approximately 1745 GMT Twitter reported that some users were still having trouble accessing the service.
Fifteen minutes later the company announced the service problems had been resolved. A company spokeswoman had no further comment.
Even during the outage, services had been restored for some affected users, only to fail again.
Twitter currently has just over 300 million users but had its slowest user growth in 2015. It was eclipsed by photo-sharing app Instagram, which is owned by Facebook Inc and surpassed 400 million users last year.
Twitter CEO Jack Dorsey's other company, Square Inc, was down almost 8 percent on Tuesday, though the reason for that was unclear.
© Thomson Reuters 2016