A bitcoin sign in a window in Toronto. Reuters/Mark Blinch Bitcoin was down by 8%, or $85, at $975 a coin as of 4:06 p.m. ET on Thursday after at least two of China's biggest bitcoin exchanges announced they were blocking customers from withdrawing their bitcoins. The announcements followed Wednesday's meeting between the People's Bank of China and the bitcoin exchanges.

Thursday's announcements are notable because nearly 100% of all bitcoin transactions take place on Chinese exchanges. The cryptocurrency has had a wild start to 2017 after gaining 120% in 2016, when it became the top-performing currency for a second straight year.

Bitcoin gained more than 20% in the opening week of 2017 before crashing by 35% on concerns China would start cracking down on trading. China's largest bitcoin exchanges recently announced they would charge a flat fee of 0.2% on all transactions.

Thursday's steep slide has pushed bitcoin to its lowest level since the final trading day of January. It is still higher by 3.6% for the year.