The DoS attacks slowed, and at times blocked, access to Nasdaq.com and the exchange operator's public market data site Nasdaqtrader.com. The Bats and Cboe public site experienced similar difficulties.

Nasdaq spokesman Joseph Christinat, told Reuters: "The Web site wasn't hacked, nobody got any information. What they did was try to block access for our users."

The perpetrators have not been identified but the Financial Times, citing people briefed on the attacks, says that a group calling itself "the 99 per cent" - a reference to the Occupy Wall Street movement - may be responsible.

Last year the Anonymous hacking collective threatened to 'erase' the New York Stock Exchange from the Internet in support of the OWS protests, although the attack failed to materialise, with the group later claiming it had been running a media scare tactic. The Nyse site has not been affected by this week's rash of DoS stings.

Yesterday Bursa Malaysia saw its Web site knocked out thanks to a DoS attack whilst last month the Tel Aviv Stock Exchange, Saudi Stock Exchange and the Abu Dhabi Securities Exchange all saw their sites disrupted as part of a wider cyber war in the Arab world.

In 2010 Nasdaq OMX was the victim of a more serious attack when hackers broke into Directors Desk, its Web-based service that lets management and board members of listed companies share confidential documents securely. An FBI investigation probing the issue found serious security lapses, with out-of-date software, misconfigured firewalls and uninstalled patches on the exchange operator's PCs.