Germany's Munich airport reopened on Friday after shutting overnight due to drone sightings that forced the cancellation or diversion of dozens of flights on the eve of a national holiday and heightened concerns about the vulnerability of critical infrastructure in Europe.

As operations resumed early on Friday morning, a Reuters witness saw passengers checking in for a flight to Varna in Bulgaria, and the departure board showed only a few flights had been cancelled. A flight from Bangkok was the first of the day to land at around 5:25am (0325 GMT), according to the airport's website.

The airport said several drone sightings late on Thursday evening had forced air traffic control to suspend operations, leading to the cancellation of 17 flights and disrupting travel for nearly 3,000 passengers, who were provided with camp beds, blankets and food.

Another 15 arriving flights were diverted to other cities including Stuttgart, Nuremberg, Vienna and Frankfurt, the airport said.

The drones were sighted in the late evening above the airport, a police spokesman told newspaper Bild. But because it was dark, the size and types of the drones could not be determined, he added. Police did not immediately respond to Reuters' requests for comment.

The disruption in Munich is the latest in a string of similar drone incidents that have rattled European aviation and raised broader security concerns, coming after airspace intrusions temporarily shut airports in Denmark and Norway last week.

The incidents prompted a sharp response from European Union leaders, who backed plans at a summit in Copenhagen on Wednesday to bolster the bloc's defences with anti-drone measures.

"Europe must be able to defend itself," Danish Prime Minister Mette Frederiksen said after the meeting.

Authorities have not publicly blamed a specific actor for the Munich drone incident, but some European officials have suggested Russia is behind other recent airspace violations.

"Russia tries to test us. But Russia also tries to sow division and anxiety in our societies," European Commission chief Ursula von der Leyen said on Wednesday.

Russian President Vladimir Putin joked on Thursday that he would not fly drones over Denmark anymore, but Moscow has denied responsibility for the incidents.

The airport disruption in Munich added to a tense week for the city after its popular Oktoberfest was closed temporarily due to a bomb threat and the separate discovery of explosives in a residential building in the city's north.

Friday is German Reunification Day, a public holiday.