There were delays on Monday at airports in London, Paris, Amsterdam, Frankfurt, Berlin, and Brussels, where visibility reportedly dropped to just 100m.
The situation was worsened at Duesseldorf Airport by the discovery overnight of an unexploded World War Two bomb.
A controlled explosion closed the airport to flights for an hour.
According to Eurocontrol, the European air safety agency, there were more than 120,000 minutes of flight delays across the continent on Monday morning, with 94% due to adverse weather conditions.
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A Eurocontrol spokesman told the Guardian it was "unusual situation to have such widespread low visibility for such a long period".
Airports in Paris, Frankfurt, Amsterdam and the UK were among the worst affected, according to the agency's Twitter feed.
Easyjet said it was suffering "major network disruption today due to poor weather conditions throughout Europe causing low visibility".
"This has resulted in a large number of aircraft and crew out of place for flights today," the airline said in a statement.
"In addition we are seeing a lot of Air Traffic Control restrictions which has resulted in some delays, diversions and cancellations."
In Paris, the top half of the Eiffel Tower was obscured by a thick blanket of fog.
Several airlines warned on Twitter that delays were expected to continue throughout Monday.
There were widespread delays at UK airports. Heathrow airport said it expected to cancel around 10% of flights on Monday, while London City was forced to close to all traffic.
Other affected airports include Manchester, Leeds Bradford, Glasgow, Belfast and Cardiff.
Heavy fog is causing disruption across western Europe, with low visibility leading to significant flight delays.