Twitter said the attacks -- in which hackers accessed names and email addresses of account holders -- were similar to recent attacks on several U.S. news organizations, CNN reported Saturday.
The New York Times, The Wall Street Journal, Bloomberg News and The Washington Post have all announced security breaches. The Times and the Journal said the attacks on their computers originated in China and were responses to coverage of Chinese news.
The Times said negative coverage of Prime Minister Wen Jiabao had provoked the the attack.
"This attack was not the work of amateurs, and we do not believe it was an isolated incident. The attackers were extremely sophisticated, and we believe other companies and organizations have also been recently similarly attacked," said Bob Lord, Twitter's security chief.
The attackers "may have had access to limited user information," Lord said.
Twitter, which has 200 million unique visitors per month, has reset the passwords of the affected accounts and canceled tokens that allow users to remain logged in to their accounts without having to use passwords.