The website of the New York Stock Exchange slowed down significantly twice on Monday afternoon, the day when the hacker group Anonymous was scheduled to launch a DDoS (distributed denial-of-service) attack on the website, according to an Internet and mobile cloud monitoring company

The news came a day after a fire at the company's data centre in New Jersey that temporarily affected communications to 58 trading firms.

It appears that the hack attack was half-hearted, and was supported by only a section of Anonymous. It also reflected the confusion that can prevail in an organization that is said not to have a centralised structure.

"We did measure the site from 10 US cities and all our measurements went south at 2:30 p.m.," said Daniel C. Berkowitz, a spokesman at Keynote Systems. Berkowitz said that the NYSE website became so slow between 2:30 pm and 3:00 pm Pacific Time "that for all intents and purposes it was unavailable/unusable by visitors".

Keynote also observed a very severe slowdown at 12:30 pm that lasted one minute, about the time that Anonymous said the attack would occur. "This did not affect trades, (but) simply the ability to get to the homepage of the New York Stock Exchange website," he added.

Keynote said it wasn't clear whether the slowing down of the NYSE site was the result of a DDoS attack.

AlertSite, owned by SmartBear Software, also reported an outage on the web site of the NYSE. The company did not immediately respond to a request for comment.

One section of Anonymous claimed that the announcement of the "Operation Invade Wall Street", which threatened an attack on the NYSE site, was a plant. "It is a fake planted operation by law enforcement and cyber crime agencies in order to get you to undermine the Occupy Wall Street movement," the group said in a message on Pastebin.

Another section within Anonymous said on Monday that the announcement had been a "high scale media scare tactic" to lull media and government ahead of big attacks.

A third group may have decided to go ahead with the attack, however. "Even though the majority does not want the operation to happen, factions of Anonymous are going for the attack," according to a comment on the original announcement on YouTube.

The Occupy Wall Street movement, supported by Anonymous, is protesting economic inequality and the greed of businesses and the financial sector in the US.

NYSE was not immediately available for comment, but according to some reports a spokesman for NYSE Euronext, the owners of the exchange, said that there were no disruptions to the website.