The Twitter accounts of several well-known figures and companies have apparently been compromised by an unknown hacker asking users to send funds to a Bitcoin account.
Obama, with over 120 million followers, has more than any other person on the platform.
Twitter said it was the victim of a "coordinated social engineering attack," in a statement late Wednesday night. It said the attackers "successfully targeted some of our employees with access to internal systems and tools.
"We know they used this access to take control of many highly-visible (including verified) accounts and Tweet on their behalf," Twitter said in a statement. "We’re looking into what other malicious activity they may have conducted or information they may have accessed and will share more here as we have it."
Before the company was able to regain control of the accounts, it told users, "You may be unable to Tweet or reset your password while we review and address this incident."
Verified accounts were not allowed to post for about two hours as a precaution.
At 8:41 p.m., hours after the initial hacking, Twitter said things should be mostly back to normal.
"Most accounts should be able to Tweet again," the company wrote. "As we continue working on a fix, this functionality may come and go. We're working to get things back to normal as quickly as possible."
In a late-night pair of tweets, Twitter wrote, "We have locked accounts that were compromised and will restore access to the original account owner only when we are certain we can do so securely. Internally, we’ve taken significant steps to limit access to internal systems and tools while our investigation is ongoing."
The hashtag "#hacked" quickly rose to the top of trends on the site Wednesday afternoon.
The apparent scam message asked users to send Bitcoin, which would then be doubled.
"I am giving back to the community. All Bitcoin sent to the address below will be sent back doubled! If you send $1,000, I will send back $2,000. Only doing this for 30 minutes," the message read on the hacked accounts.
Biden's campaign quickly responded to its account being hacked.
“Twitter locked down the account immediately following the breach and removed the related tweet. We remain in touch with Twitter on the matter,” the campaign said in a statement to ABC News.
The Department Of Homeland Security’s cyber arm, the Cybersecurity and Infrastructure Security Agency, referred questions to Twitter and said it had no further comment.
FBI's San Francisco field office, which covers Twitter's headquarters, offered a statement.
"We are aware of today’s security incident involving several Twitter accounts belonging to high profile individuals," it said. "The accounts appear to have been compromised in order to perpetuate cryptocurrency fraud. We advise the public not to fall victim to this scam by sending cryptocurrency or money in relation to this incident."
A Secret Service spokesperson told ABC News, "The U.S. Secret Service is aware of a suspected hacking incident related to social media accounts. As a matter of practice, the Secret Service does not confirm the existence or absence of ongoing investigations."
Rep. Alexandria Ocasio-Cortez, D-N.Y., immediately warned people not to take part in the scam.
ABC News' Molly Nagle, Luke Barr, Catherine Thorbecke and Marc Nathanson contributed to this report.