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."
"Tough day for us at Twitter. We all feel terrible this happened," tweeted CEO Jack Dorsey late on Wednesday.
"We're diagnosing and will share everything we can when we have a more complete understanding of exactly what happened," said Mr Dorsey.
Earlier, Twitter took the extraordinary step of stopping many verified accounts marked with blue ticks from tweeting altogether.
There were reports that password reset requests were also being denied.
San Francisco-based Twitter later said most accounts would be able to tweet again, but the company 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.
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."
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."