"We're seeing surges in a variety of countries which will likely weigh on the recovery in the coming months, just as it was starting to gather pace."
London's FTSE 100 ended the day back in the black, edging up by around 0.1 percent, while stock prices in both Frankfurt and Paris were weaker.
On the other side of the Atlantic, Wall Street was in the red as investors monitored progress on a large infrastructure package nearing approval in Washington, traders said.
Commodities such as oil "have been hurt by growing concerns over the Delta variant of the coronavirus," said ThinkMarkets analyst Fawad Razaqzada.
"Rising cases and more lockdowns have had a direct impact on travel and thus oil demand recovery."
Oil prices were down by around two percent on Monday.
The dollar was mixed against its biggest rivals.
On Wall Street, the Dow and S&P 500 -- which had ended last week at records following strong jobs data -- were both negative on Monday.
Key reports this week include an inflation read from the consumer price index on Wednesday, as well as earnings from Disney on Thursday.
Also in investors' sights will be the possible approval in the Senate of a $1.2-trillion infrastructure bill backed by President Joe Biden after it cleared a key procedural vote on Sunday.
On Monday, Bitcoin steadied after surging more than 20 percent over the weekend to more than $45,000.
Analysts said cryptocurrencies were enjoying a recovery after weeks of being buffeted by Chinese moves to crack down on the sector.