Hong Kong-based crypto trading platform OKEx will roll back futures transactions following what it called an "irregular" sell-off.

OKEx, operated by cryptocurrency exchange OKCoin, said Friday that the move comes after it temporarily halted futures trading and token withdrawals following the incident, which occurred between 5 a.m. and 6:30 a.m. Hong Kong Time (HKT) or 21:00 to 22:30 UTC this morning. The sell-off pulled down the price of the quarterly bitcoin futures significantly compared to the cryptocurrency's value.

The rollback will undo transactions going back to 04:47 a.m. HKT, and is set to occur at 3:30 p.m. HKT today, according to the statement.

"OKEx always has the customers' best interests at heart, and we are dedicated to providing the best products and technologies to protect our customers. Transactions were suspended for several hours due to the incident. Please accept our sincere apologies for the inconvenience caused," the platform's team wrote in its notice.

The announcement came in the wake of widespread criticism from users, who complained that OKEx's system can't prevent what was seen by many as maliciously intended activities. Some investors went as far as to allege that OKEx itself was involved in the alleged manipulation. OKEx has not responded to the comments.

Screen captures

shared on Twitter show that during the period, the price of bitcoin futures on OKEx first started the fall around 22:00 UTC and dropped to nearly $5,200. A quick rebound pushed prices to around $6,000, only to slide further to as low as $4,755.

However, within the same period, the price of bitcoin itself largely held at around $7,000, according to CoinDesk data, indicating a 20 percent difference between prices.

The price of bitcoin fell to a 50-day low Friday morning as it dropped below $7,000. During Thursday's trading session, the prices of other major cryptocurrencies, including bitcoin cash and ether, fell to their lowest point since the start of the year.

Trading board image via Shutterstock