Nordic stocks fell sharply, by 8%, Monday before partly recovering in what brokerage Nordnet said was a "Flash Crash" triggered by a brief market panic.

A single sell order trade by Citigroup Inc. that reversed digits, known as a "fat finger" trade, caused a five-minute, steep selloff on a day of London markets holiday-thinned trading, Reuters reports.

"The reason for the drop was a sell event by a market participant," a Nasdaq spokesperson in Stockholm told Reuters in an emailed statement May 2 before market close. "We have not identified any disturbances in Nasdaq's systems. Furthermore, after a review, Nasdaq has not seen any reason to cancel trades that were made during this event."

The Stockholm benchmark stock index OMX 30 was trading down 2.5% at 0854 GMT, while the bourses in Copenhagen and Oslo also experienced big falls.

Other European markets also tumbled, including in Germany, Italy and France, but later partly recovered. The Nordic trade caused the pan-European STOXX 600 equity benchmark index to fall more than 2 percentage points for about two minutes starting at 0758 GMT.

"It was weird, in those minutes there," Martin Munk, equity sales vice president at Jyske Bank, told Reuters. Munk added he took calls from worried clients in those critical few minutes.

Earlier, on Monday, a Euronext Oslo spokesperson said there was no news in the market that could explain the rapid decline.

"We are looking into this as a matter of routine; there was no news in the market that could explain such a large move," Euronext Oslo spokesperson Cathrine Segerlund said.

Nasdaq, which operates the stock exchanges in Stockholm and Copenhagen, was not immediately available for comment when contacted by Reuters.

A Flash Crash occured in the U.S. on May 6, 2010, wiping out $1 trillion in market valuations for 36 minutes and causing the Dow Jones Industrial Average to plunge 998.5 points, or 9%. The brief crash, which corrected after circuit breakers were imposed to shut the markets down, was reportedly caused by derivatives algorithms.