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 the first such tweet from Musk's account was removed, another one appeared, then a third.
Others targeted included:
the rapper Kanye West
his wife, reality TV star Kim Kardashian
former US President Obama
former US Vice-President Joe Biden, who is the current Democratic presidential candidate
media billionaire Mike Bloomberg
the ride-sharing app Uber
the iPhone-maker Apple
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 [email protected]
The name "Anthony Elias" was used to register the website, but may be a pseudonym.
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."