Trading on the MTGox exchange, which handles most trades in Bitcoins, was sluggish yesterday as the site fought off an attack.
The attack helped to force a swift fall in the price of Bitcoins.
In addition, the Instawallet website - where people store Bitcoins - is offline indefinitely after an attack.
The value of Bitcoins surged to a new high this week with each one worth about $142 (£94). Barely a week ago, each virtual coin was worth only $90.
But Bitcoins dropped sharply in value as the MTGox exchange came under a sustained attack by hackers. The vast majority of trade in Bitcoins takes place via the site.
In a tweet on its Twitter feed, MTGox said it was fighting off a distributed denial-of-service (DDoS) attack, which involves a site being bombarded with huge amounts of data. The attack was one of several against the site this week,
The attacks, coupled with a spike in trading volumes, combined to cause delays in trades being confirmed and led the value of Bitcoins to drop sharply to about $120.
The attacks could be the work of malicious hackers who were trying to 'game' the MTGox exchange
The attacks could be the work of malicious hackers who were trying to "game" the exchange and manipulate the value of Bitcoins so they could cash in, MTGox said in an interview with ComputerWorld. Attackers are thought to be working to a cycle in which they sell Bitcoins when values are high, then mount an attack that forces prices to crash, buy up the cheaper coins and then let the value climb again.
MTGox said it did not know when or if the attacks would cease but said Bitcoin owners should not panic and sell off as values fluctuated. A spokesman for the exchange added that it was in the middle of rebuilding its trading technology but the new system, which would do a better job of handling the high volume of trades, would not be ready until the end of this year.
In a separate development, Instawallet has shut down "indefinitely" after hackers "fraudulently accessed" its core database. In a statement posted on the Instawallet site it said it planned to open a claim process shortly so people could reclaim their Bitcoin balance.