Twitter's information-security director Bob Lord said the users' passwords had been stolen, as well as usernames, emails and other data.
Lord said the attack "was not the work of amateurs". He said it appeared similar to recent attacks on the New York Times and the Wall Street Journal. The US newspapers reported their computer systems had been breached by China-based hackers.
He added: "The attackers were extremely sophisticated, and we believe other companies and organisations have also been recently similarly attacked.