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.
Cancelled flights: Why does fog cause chaos at airports?
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.

In an open letter to Apple, Swift said she was withholding the record as she was unhappy with the three-month free trial offered to subscribers.
"I'm not sure you know that Apple Music will not be paying writers, producers, or artists for those three months," she wrote.
She said the plan was "unfair", arguing Apple had the money to cover the cost.
"I find it to be shocking, disappointing, and completely unlike this historically progressive and generous company," the 25-year-old said, describing Apple as one of her "best partners in selling music".
"These are not the complaints of a spoiled, petulant child. These are the echoed sentiments of every artist, writer and producer in my social circles who are afraid to speak up publicly because we admire and respect Apple so much," she continued.
"We know how astronomically successful Apple has been and we know that this incredible company has the money to pay artists, writers and producers for the 3 month trial period - even if it is free for the fans trying it out.
"Three months is a long time to go unpaid, and it is unfair to ask anyone to work for nothing."
She ended her letter by calling on Apple to change its policy, suggesting she would reinstate her album on the service if the company changed its mind.
"We don't ask you for free iPhones. Please don't ask us to provide you with our music for no compensation."
Apple has yet to comment on the issue.
The rest of Swift's back catalogue will remain available on Apple Music when it launches on 30 June. It will cost $9.99 (Â£6.30) per month in the US for one person or $14.99 for families.
The music industry has generally supported the company bringing its vast music library to paid streaming, and the company said it will pay 70% of the music subscription revenue to music owners.
It is not the first time Swift has spoken out against streaming music - she pulled her entire catalogue from Spotify last November and had refused to offer 1989 on streaming services, saying the business had "shrunk the numbers of paid album sales drastically".
1989 went on to become one of the biggest-selling albums of 2014 and has sold more than 4.9 million copies in the US alone.
Last week, UK-based independent record label Beggars echoed Swift's comments, saying it struggled "to see why rights owners and artists should bear this aspect of Apple's customer acquisition costs".
It said it did not have an agreement with Apple that would allow it to participate in the new service but hoped the "obstacles to agreement can be removed" in the coming days.
Heavy fog is causing disruption across western Europe, with low visibility leading to significant flight delays.
[NEXT_CONCEPT]
Taylor Swift has pulled her hit album 1989 from Apple's new streaming music service and criticised the company.