KEY POINTS

U.S. banking giant Citigroup on Monday took responsibility for the flash crash which saw some European stocks turn sharply lower.

Nordic stocks were hit the hardest, with Sweden's Stockholm OMX 30 share index falling by as much as 8% at one point, before paring most of those losses to close the session down 1.9%.

"On Monday, one of our traders made an error when inputting a transaction. Within minutes, we identified the error and corrected it," a spokesperson for Citi told CNBC.

A so-called flash crash in European markets on Monday prompted several indexes to tumble sharply, sparking alarm among investors on a day when trading was thin due to public holidays around the world.