• The Stockholm OMX 30 fell 8% at around lunch time before quickly paring losses just minutes afterward.
• The U.K.'s FTSE 100 was closed for a public holiday and trading volumes were impacted.
• German retail sales out Monday morning showed an unexpected fall in March. The Federal Statistics Office said sales were down 0.1% for the month in real terms.

LONDON — European markets started the month of May on a weak footing, with investors digesting fresh economic data and a sudden fall in the Swedish stock market.

The German DAX closed down by 1.4% while the French CAC finished 2% lower. The U.K.'s FTSE 100 was closed for a public holiday and trading volumes were impacted.

The Stockholm OMX 30 fell 8% at around lunch time before quickly paring losses just minutes afterward. Reuters later reported, citing people familiar with the matter, that a flash crash had been caused by a single sell order trade by Citigroup Inc. It reportedly involved a wrong calculation relating to a Nasdaq Inc index involving Swedish stocks, and briefly weighed on the wider indexes.

Meanwhile, German retail sales out Monday morning showed an unexpected fall in March. The Federal Statistics Office said sales were down 0.1% for the month in real terms.

Global investors continue to monitor the war in Ukraine and its geopolitical implications. EU leaders are set to work on a Russian oil embargo this week. Over the weekend, the United Nations and the International Committee of the Red Cross began evacuating civilians from the besieged southern port city of Mariupol. That operation is set to continue on Monday.

On Wall Street, stocks rose slightly on Monday morning after the Nasdaq Composite Index posted its worst month since 2008, pressured by rising rates, rampant inflation and underwhelming earnings from some of the largest technology companies.

Shares in Asia-Pacific slipped on Monday, with data released over the weekend showing Chinese factory activity contracted in April.

Back in Europe, shares of wind turbine firm Vestas were down 7.8% after cutting its guidance. Airbus shares were down 1.8% despite taking a large order from airline Qantas for its London-Sydney route.

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