"A SPOOKY fog creeping slowly across Melbourne is a rare freak of nature, the weather bureau says.

The dense fog covered large sections of Melbourne and was affecting flights at Melbourne Airport.

Passengers are advised to check with their airlines or the Melbourne Airport website after a number of diversions and delays.

The fog was expected to cover much of the metropolitan area tonight, after visibility was reduced to 500m at Moorabbin Airport earlier this evening.

"So much fog, I can't even see if the tram is coming," one Twitter user wrote.

Another tweeted that the eerie sight looked "like Armageddon rolling in"."