Danish police on Tuesday said whoever was responsible for flying large drones over Copenhagen airport appeared to have been knowledgeable, as flights resumed following a night of travel chaos.

Airports in Copenhagen and Oslo reopened on Tuesday, hours after unidentified drones in their airspace caused dozens of flights to be diverted or cancelled, disrupting thousands of passengers.

Heavy travel delays were expected to last throughout Tuesday.

Copenhagen police inspector Jens Jespersen told reporters that "the number, size, flight patterns, time over the airport. All this together ... indicates that it is a capable actor. Which capable actor, I do not know."

He said "several large drones" flew over the Copenhagen airport for more than three hours late on Monday.

A heavy police presence was dispatched to investigate the drone activity, and the devices could be seen coming and going for several hours before flying away on their own.

Police were cooperating with the Danish military and intelligence service in their investigation, Jespersen said.

He said police chose to not shoot down the drones for safety reasons.

"You have to think very carefully before starting to try to take down such big drones."

If they were to fall to the ground, "there are planes with people, fuel, and also housing on several sides of the airport," he said.

Jespersen said it was not known where the drones were being controlled from, but that it could have been from many kilometres away.

The drones were flying from several directions, he said.

"It could very well be something initiated from a ship," he told Danish broadcaster DR. (AFP)