"This attack was not the work of amateurs, and we do not believe it was an isolated incident," Twitter information security director Bob Lord said in a blog post.
Lord referred to an "uptick in large-scale security attacks aimed at US technology and media companies" as he told of Twitter detecting attempts this week to get unauthorized access to data in the firm's network.
The attack coincided with the revelation of several high-profile security breaches. The New York Times and The Wall Street Journal said this week that they had been hacked, and pointed to hackers from China.
Twitter did not confirm the source of the intrusion.
But Lord noted that "the attackers were extremely sophisticated, and we believe other companies and organizations have also been recently similarly attacked."
He said that Twitter shut down a live attack as it was in process.
However, Lord added, cyber attackers may have gotten usernames, email addresses, passwords and other data.
As a precaution, Twitter invalidated passwords of accounts at issue and sent people email messages telling them to create new passwords.