The exchange, Bitfinex, responded by halting trading, deposits and withdrawals, prompting a plunge in the Bitcoin price.
The hackers made off with 119,756 bitcoins, said Zane Tackett, Bitfinex’s director of community and product development, in an email to CNNMoney. That’s the equivalent of more than $65 million at current prices.
The Hong Kong exchange said it has reported the hacking to law enforcement. It gave no information about how the attack happened, or who may have been responsible.
Tackett told CNNMoney that Bitfinex is working with authorities and specialist companies to track the stolen bitcoins. It’s also aiming to get its platform up and running again so that users can see if their accounts were hit.
The price of Bitcoin tumbled more than 20% following the news, before recovering some of its losses.
The security breach brought back memories of Mt.Gox, a leading Bitcoin exchange that stopped investors from withdrawing money in 2014, blaming the disruption on technical issues and cyber attacks. It later filed for bankruptcy.
Bitfinex is one of the biggest Bitcoin exchanges in the world. It had the highest volume of dollar-denominated transactions over the past 30 days, according to the website Bitcoin Charts.