"Unfortunately, there has been a security breach involving NiceHash website", the company wrote, adding that it was freezing all operations for at least the next 24 hours while it investigated the hack.
The company has reported the incident to police and told users to change their passwords. While the company did not reveal the exact amount lost, users on Reddit spent the better part of today anxious as the service was initially "under maintenance" for 12 hours amidst suspicious transactions on the blockchain that saw 4,736.42 BTC taken from Nicehash and their Nicehash internal wallets reporting zero balances. Mt. Gox shut down in February 2014, saying it lost about 850,000 bitcoins, possibly to hackers. In addition to undertaking our own investigation, the incident has been reported to the relevant authorities and law enforcement and we are co-operating with them as a matter of urgency.
Although NiceHash has not confirmed the number of bitcoins stolen, NiceHash users have circulated a wallet address that suggests that 4,736.42 BTC - worth more than $62 million at the current exchange rate - was drained from the company's hot wallet.
Bitcoin wallets are usually a highly secure technology, but the hack may have included a sophisticated attempt to scam and steal wallet keys.
Troubles with the website over the past day or so drew alarm and complaints, with many bitcoin owners posting panicked comments on NiceHash's social media accounts.
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CoinDesk will continue monitoring this developing story. On Jan. 1 this year, bitcoin was priced at just under $1,000.
Bitcoin is the world's most popular digital currency.
Online security is a vital concern for cryptocurrency marketplaces and exchanges, with bitcoins contained within digital wallets that have increasingly become a target for hackers as the number of bitcoins stored and their value has skyrocketed over the previous year.
The digital currency's value has soared in recent weeks, crossing $14,000 for the first time on Thursday morning in Asia. It also allows NiceHash users to purchase computer processing power from other users.