Billionaires Elon Musk, Jeff Bezos and Bill Gates are among many prominent US figures targeted by hackers on Twitter in an apparent Bitcoin scam.
The official accounts of Barack Obama, Joe Biden and Kanye West also requested donations in the cryptocurrency.
“Everyone is asking me to give back,” a tweet from Mr Gates’ account said. “You send $1,000, I send you back $2,000.”
Twitter said it was a “co-ordinated” attack targeting its employees “with access to internal systems and tools”.
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“We know they [the hackers] used this access to take control of many highly-visible (including verified) accounts and Tweet on their behalf,” the company said in a series of tweets.
It added that “significant steps” were taken to limit access to such internal systems and tools while the company’s investigation was ongoing.
Meanwhile, Twitter CEO Jack Dorsey tweeted: “Tough day for us at Twitter. We all feel terrible this happened.”
Emergency response
Twitter earlier had to take the extraordinary step of stopping many verified accounts marked with blue ticks from tweeting altogether.
Password reset requests were also being denied and some other “account functions” disabled.
Users with verified account started to be able to send tweets again, but Twitter said it was still working on a fix.
Dmitri Alperovitch, who co-founded cyber-security company CrowdStrike, told Reuters news agency: “This appears to be the worst hack of a major social media platform yet.”
On the official account of Mr Musk, the Tesla and SpaceX chief appeared to offer to double any Bitcoin payment sent to the address of his digital wallet “for the next 30 minutes”.
“I’m feeling generous because of Covid-19,” the tweet added, along with a Bitcoin link address.
The tweets were deleted just minutes after they were first posted.
Cameron Winklevoss, who was declared the world’s first Bitcoin billionaire in 2017 along with his twin brother Tyler, tweeted a message on Wednesday warning people not to participate in the “scam”.
In the short time it was online, the link displayed in the tweets of targeted accounts received hundreds of contributions totalling more than $100,000 (£80,000), according to publicly available blockchain records.
The Twitter accounts targeted all have millions of followers.
Last year, Twitter chief executive Jack Dorsey’s account was hacked, but the company said it had fixed the flaw that left his account vulnerable.
The FBI’s San Francisco field office put out a statement on Wednesday about the latest cyber-breach.
“The accounts appear to have been compromised in order to perpetuate cryptocurrency fraud,” it said.
“We advise the public not to fall victim to this scam by sending cryptocurrency or money in relation to this incident.”