The drone incursion that stopped flights at Copenhagen airport on Monday night was the most severe attack on Danish infrastructure so far, Denmark's Prime Minister Mette Frederiksen said.
Kastrup airport in Copenhagen was forced to shut for several hours from around 20:30 (18:30 GMT) on Monday following the sighting of a number of drones.
It says something about the times we live in and what we as a society must be prepared to deal with, Frederiksen told reporters.
Russian involvement could not be ruled out, Frederiksen added - although Kremlin spokesman Dmitry Peskov called the allegations unfounded.
The Danish PM made a link between last night's events in Denmark and the recent Russian drone incursions in Poland and Romania, as well as the violation of Estonian airspace by Russian fighter jets.
Frederiksen said that the motive for the incursion in Copenhagen had likely been to disrupt, create unrest... to see how far you can go and test the limits.
Danish intelligence mirrored this assessment, saying the country was facing a high threat of sabotage.
In nearby Norway, Oslo airport too was closed for a period after possible drone sightings. Approximately 20,000 passengers were impacted by the closure of Copenhagen airport, which resumed operations after midnight local time.
Danish police stated they did not know who was behind the drones, but that evidence suggested it was a capable actor. The drones were observed coming from different directions and were operated by someone with the will and tools to show off... perhaps also to practice.
Tensions have been escalating recently after Russian drones and aircraft ventured into central and eastern European airspace. Earlier in September, at least three Russian drones were shot down by Polish and other Nato aircraft in Poland's airspace. Moscow denied it had been trying to target facilities on Polish soil.
No matter the cause, security in European airspace has become a priority for NATO, which is responding by bolstering its Eastern defenses with military assets from multiple member nations.