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”.
“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.”
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.
By 20:30 EDT (00:30 GMT Thursday) 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.
But as such tweet from Musk’s account was removed, another one appeared, then a third.
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As well as rapper Kanye West, his wife, reality TV star Kim Kardashian, former US President Obama, Mr Biden, who is the current Democratic presidential candidate, and media billionaire Mike Bloomberg, major companies Uber and Apple were targeted.
The Biden campaign said Twitter had “locked down the account within a few minutes of the breach and removed the related tweet”.
A spokesman for Bill Gates told AP news agency: “This appears to be part of a larger issue that Twitter is facing.”
The BBC can report from a security source that a web address – cryptoforhealth.com – to which some hacked tweets directed users was registered by a cyber-attacker using the email address firstname.lastname@example.org.
The name “Anthony Elias” was used to register the website.
Cryptoforhealth is also a registered user name on Instagram, apparently set up contemporaneously to the hack.
The description of the profile read “It was us”, alongside a slightly smiling face emoticon.
The Instagram profile also posted a message that said: “It was a charity attack Your money will find its way to the right place.”
An unprecedented ‘smash and grab’
These “double your Bitcoin” scams have been a persistent pest on Twitter for years but this is unprecedented with the actual accounts of public figures hijacked and on a large scale.
The fact that so many different users have been compromised at the same time implies that this is a problem with Twitter’s platform itself.
Early suggestions are that someone has managed to get hold of some sort of administration privileges and bypassed the passwords of pretty much any account they want.
With so much power at their fingertips the attackers could have done a lot more damage with more sophisticated tweets that could have harmed an individual or organisation’s reputation.
But the motive seems to be clear – make as much money as quickly as they can. The hackers would have known that the tweets wouldn’t stay up for long so this was the equivalent of a “smash and grab” operation.
There are conflicting accounts of how much money the hackers have made and even when a figure is settled upon, it’s important to remember that cyber-criminals are known to add their own funds into their Bitcoin wallets to make the scam seem more legitimate.
Either way, it’s going to be very hard to catch the criminals by following the money. Law enforcement, as well as many angry users, will have some strong questions for Twitter about how this could have happened.
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.”