250,000 Twitter users have had their accounts hacked. Passwords, usernames, emails and other data have all been compromised.
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Those who have been affected have had their passwords invalidated and have been emailed by Twitter informing them of the issue.
However, a major concern for all Twitter users is not just the vulnerability of their account, but the additional new phishing scams that the attack has led to.
According to the site's Information Security Director Bob Lord, the attack "was not the work of amateurs", adding that it appeared similar to recent attacks on US newspapers the New York Times and the Wall Street Journal, who reported that their computer systems had been breached by China-based hackers.
Twitter has 200 million active users worldwide, and it is still not clear as to the exact nature of the attack, or why just one small section of users is endangered.
Mr Lord added: "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."
Aware of the problem, Twitter users who are now on the lookout for a warning email from Twitter are already being targeted by fraudsters with scam emails – purporting to be from the site – instructing them to click on links to change their passwords. The links are malicious and unsuspecting victims can then find that they have malware on their machines.
Our advice is to go to the Twitter site as usual and try and log in to your account. If there is a problem, you will then need to try to reset your password if it has been invalidated.
And as always, never click on links contained in emails, but enter the website address you know to be correct in your browser.
On Thursday the New York Times linked the attack to a story it published alleging relatives of former Premier Wen Jiabao controlled assets worth billions of dollars.
China's foreign ministry dismissed the New York Times' accusations as "groundless" and "totally irresponsible".
