"The police have launched an intensive investigation to determine what kind of drones these are," Copenhagen Police Deputy Assistant Commissioner Jakob Hansen told reporters.
"The drones have disappeared and we have not taken any of them," he added.
Hansen said authorities in Denmark and Norway would co-operate to determine whether there was a link between the two incidents.
Oslo Airport shut its airspace due to a drone observation, a spokesperson for Norwegian airport operator Avinor said in an emailed statement, adding that all flights were diverted to the nearest airport.
Danish police said earlier that two or three large drones had been seen flying near Copenhagen's airport, closing it to all traffic.
The airport halted operations at 4.26am AEST on Tuesday, according to flight tracking service FlightRadar.
Around 50 flights were diverted to alternate airports, FlightRadar said on X.
After it reopened, Copenhagen Airport said on X that delays and some cancelled departures would persist and urged passengers to check with their airlines.
The airport shutdowns came after a string of disruptions at European airports in recent days.
A cyberattack last Friday knocked out check-in and boarding systems supplied by Collins Aerospace, a unit of RTX, affecting operations at London's Heathrow and the Berlin and Brussels airports. Over the weekend and into Monday, the fallout continued to snarl travel across Europe.
In 2018, drone sightings over the runway at Gatwick near London stranded tens of thousands of passengers and disrupted hundreds of flights at the height of the holiday season.
