Tens of thousands of Twitter users, including the BBC's technology correspondent, have had their accounts hacked.
In a blog post, staff at the micro-blogging website said that earlier this week it interrupted attempts to access user data, and that attackers stole 250,000 users' login names, email addresses and encrypted passwords.
It shut down one attack moments after it was detected.
Twitter reset the compromised passwords and sent emails informing affected users.
One of those users was Rory Cellan-Jones, a journalist who covers technology for the BBC.
The attack follows other high-profile hacking attacks, inlcluding China-based hackers' inflitrating computer systems at The New York Times and The Wall Street Journal, and Anonymous's hijack of the US Sentencing Commission's website.
Twitter said in its blog that the 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 organisations have also been recently similarly attacked.
"For that reason we felt that it was important to publicise this attack while we still gather information, and we are helping government and federal law enforcement in their effort to find and prosecute these attackers to make the internet safer for all users."
One expert said that the Twitter hack probably happened after an employee's home or work computer was compromised through vulnerabilities in Java, a commonly used computing language whose weaknesses have been well publicized.
Ashkan Soltani, an independent privacy and security researcher, told Associated Press such a move would give attackers "a toehold" in Twitter's internal network, potentially allowing them either to sniff out user information as it traveled across the company's system or break into specific areas, such as the authentication servers that process users' passwords.
He said that the relatively small number of users affected suggested either that attackers were not on the network long or that they were only able to compromise a subset of the company's servers.
Bob Lord, Twitter's director of security, said: "We encourage all users to take this opportunity to ensure that they are following good password hygiene, on Twitter and elsewhere on the internet.
"Make sure you use a strong password – at least 10 (but more is better) characters and a mixture of upper- and lowercase letters, numbers, and symbols."