Jack Dorsey said: "We all feel terrible this happened".
A massive hack - described as "a coordinated social engineering attack" by Twitter - targeted multiple high-profile users last night including ex-US President Barack Obama, Tesla's Elon Musk, investor Warren Buffet and Amazon's Jeff Bezos. Twitter CEO Jack Dorsey responded to the attack this morning, saying it was a "tough day for us at Twitter".
"We all feel terrible this happened. We're diagnosing and will share everything we can when we have a more complete understanding of exactly what happened," Mr Dorsey tweeted.
In an extraordinary measure, Twitter disabled the validated accounts from posting tweets for about two hours after scammers trying to dupe people into sending cryptocurrency bitcoin hijacked the users' profiles.
A tweet that appeared on Elon Musk's feed read: "Happy Wednesday! I am giving back Bitcoin to all of my followers. I am doubling all payments sent to the Bitcoin address below. You send 0.1 BTC, I send 0.2 BTC back!" The offer was valid "only for 30 minutes".
"This is a SCAM, DO NOT participate!" Gemini cryptocurrency exchange co-founder Cameron Winklevoss warned from his official account on Twitter.
In a series of posts, the microblogging site later said that most accounts - that were targeted - "should be able to tweet again".
"We detected what we believe to be a coordinated social engineering attack by people who successfully targeted some of our employees with access to internal systems and tools," one of the posts read.
Among other high profile accounts that were affected were those of rapper Kanye West, Microsoft co-founder Bill Gates, and the corporate accounts for Uber and Apple.
Several accounts of cryptocurrency-focused organizations were also hijacked. Altogether, the affected accounts had tens of millions of users.
This is not the first time that Twitter has reported a massive hack. In March 2017, the accounts of Amnesty International, the French economics ministry and the BBC's North America service were broken into by hackers believed to have been loyal to Turkish President Recep Tayyip Erdogan.
Last August, a series of insulting or racist messages were posted on the personal account of Twitter founder Dorsey without his knowledge.
(With inputs from Reuters, AFP)