Twitter was hacked earlier this week and details of approximately 250,000 passwords stolen, the social network says in an official blog post.
Many people noticed problems with Twitter on Friday. The social network was often inaccessible, with the signature "fail whale" frequently appearing.
It now transpires the service was grappling with a live attack.
"Our investigation has thus far indicated that the attackers may have had access to limited user information – usernames, email addresses, session tokens and encrypted/salted versions of passwords – for approximately 250,000 users," the blog post says.
Affected users' passwords have been reset by Twitter, and they have been informed by email.
Twitter has around 200 million active users.
"This attack was not the work of amateurs, and we do not believe it was an isolated incident," Bob Lord, Twitter's director of information security, wrote in the blog post. "The attackers were extremely sophisticated, and we believe other companies and organizations have also been recently similarly attacked."
The attack on Twitter follows on the heels of attempts to infiltrate the Wall Street Journal, Washington Post and New York Times' computer systems. In the Times' case, it appears Chinese hackers targeted the paper after its investigation, four months ago, into the billions accumulated in various business deals by Chinese Prime Minister Wen Jiabao and his family.
The motivation for the Times' attack was apparently a bid to steal reporters' passwords.
The Wall Street Journal says Chinese hackers were attempting to monitor it China coverage.
Chinese Embassy spokesman Geng Shuang condemned allegations of Chinese cyberspying, the Journal says.
"It is irresponsible to make such an allegation without solid proof and evidence," he said. "The Chinese government prohibits cyberattacks and has done what it can to combat such activities in accordance with Chinese laws." He said China has been a victim of cyberattacks but didn't say from where.