Some 6,500 people had to be evacuated on Friday morning from the security zone of an airport in western Germany after a passenger had set off an alarm by going through an emergency door.

Forty-two of the 634 take-offs and landings scheduled for Friday at the Dusseldorf airport had been canceled by midday (1100 GMT), and many more were delayed.

Police said initial investigations showed that a man opened the door at the airport by mistake and was followed by two passengers.

Many travelers who had already gone through the security checks had to return to the public area of the airport, and some even had to leave their planes after boarding, according to an airport spokesman.

After two hours the gates could be reopened, although long queues had formed at the security control points, he added.

Nobody was arrested in relation to the incident, a police spokeswoman said. The door appears to have been opened unwittingly.

When an emergency door opens, a standard procedure is triggered whereby all gates are closed, the airport said.

Dusseldorf airport is the third largest in Germany. Some 75,000 passengers were expected to travel on Friday.