About 4,000 pilots at German airline Lufthansa have gone on strike for four days in a dispute over job security.
The carrier has cancelled about 3,000 flights and has warned of delays both domestically and internationally.
The strike has disrupted thousands of passengers around the world, while the company tries to arrange alternative travel for them.
The pilots work at Lufthansa, Lufthansa Cargo and budget subsidiary, Germanwings.
Lufthansa's offer of negotiations with the pilots was not taken up on Sunday before the strike began at midnight (2300 GMT).
The airline normally offers about 1,800 flights daily - of which 160 are long-haul trips.
Lufthansa has said about two-thirds of flights will have to be cancelled during the strike.
It was also reported that the company had asked a German court to halt the strike.
Lufthansa spokeswoman Claudia Lange told the Associated Press that an injunction had been filed in Frankfurt.
"This strike is disproportionate," she said. "We hope for a decision within the next 24 hours."
Before the strike began, German Transport Minister Peter Ramsauer warned that it would hurt the country's economy as well as Lufthansa's reputation.
The airline - one of the world's largest - fears the strike could cost it about 25m euros (£21.9m; $34m) per day.
The pilots are not due to return to work until midnight on Thursday.
The Cockpit union says the airline is increasingly relying on foreign pilots who fly for less pay.
It is concerned that the company could try to cut staff costs by shifting jobs to foreign subsidiaries such as Austrian Airlines or Lufthansa Italia, where wages are lower.
The starting salary for a first officer in a Lufthansa cockpit is 62,000 euros, and 115,000 euros for a captain, according to the company's recruiting website. Media reports say pilots' salaries can rise up to about 325,000 euros.
Cockpit has called for a 6.4% pay rise for pilots, more say in company decisions and commitments that pilots would keep their jobs when Lufthansa moves passengers to cheaper foreign affiliates.
But Andreas Bartels from the airline told the BBC the pilots' fear that their jobs would be outsourced was unfounded.
"That's what they fear but that's not reality. If you look to the reality, it's nothing like replacing or transferring jobs to other companies or other airlines [in] the Lufthansa airline system," he said.
He added that Lufthansa would not be dictated to by the unions.
"It's not about money. The unions made it very clear that they're willing to make a compromise when it comes to their claim on money, but it's about political influence on the company's strategy and that is something that we can't accept."
Lufthansa was offering train journeys to domestic air travellers, and attempting to rebook international passengers on other airlines.
Frankfurt and Munich airports will be the worst hit.
Hours before the strike began, Lufthansa made a last-ditch effort to resume talks.
"We are open to talks without preconditions," Lufthansa spokesman Klaus Walther said.
He added that "if Cockpit withdrew its list of unworkable and illegal demands... an agreement could be found quickly".
But Cockpit spokesman Joerg Handwerg said: "The Lufthansa management is not interested in dialogue, but will not say so openly."
Like most other global airlines, Lufthansa has struggled in the downturn. Sales slumped 13% in the first nine months of 2009.
As well as suffering from the more recent drop in consumer demand, the airline has also been facing the long-running threats posed by low-cost airlines and high fuel costs.
The UK's flagship carrier British Airways has also been locked in a dispute with cabin crew over pay and changes to working conditions.
The result of a strike ballot by the Unite union is expected at about 1700 GMT on Monday.
In France, air traffic controllers are planning a four-day strike at Paris' Charles de Gaulle and Orly airports.
Unions have called for the strike from Tuesday to Friday in protest at plans to integrate European air traffic control, fearing it would lead to a loss of French jobs and civil servant benefits.