Lufthansa has cancelled almost 900 flights after it lost an eleventh-hour legal bid to halt a pilots strike, BBC says.
The two-day strike over pay began at midnight local time, with Lufthansa saying about 100,000 passengers will be affected.
Flights by Lufthansa's other airlines including Eurowings, Swiss, Austrian Airlines, and Brussels Airlines are not affected, the airline said.
The industrial action is part of a long-running pay dispute at Lufthansa.
The pilots' union, Vereinigung Cockpit, has organised 14 strikes since April 2014.
On Tuesday, Lufthansa made two legal challenges to halt the strike.
A Frankfurt labour court first rejected Lufthansa's application for an injunction. Later in the evening, the Hesse state labour court rejected the airline's appeal.
Pay talks between the Vereinigung union and the German airline broke down earlier this month, and Lufthansa said the union had "consistently rejected the offer" of mediation.
The union is calling for a 3.7% pay rise for 5,400 pilots dating back to 2012.
Lufthansa, which is facing increasing competition from budget rivals, offered a 2.5% increase over the six years until 2019.
Meanwhile, a separate dispute with cabin crew at Lufthansa's low-cost subsidiary, Eurowings, led it to cancel more than 60 flights on Tuesday.
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