Half of flights serving France's big cities have been cancelled, as air-traffic controllers begin a three-day strike to protest against EU plans to create a single European airspace.
The controllers say the Single European Sky (SES) project will affect public safety and their working conditions.
Walk-outs are expected to spread across Europe on Wednesday.
France's civil aviation authority says one in two flights to Paris, Lyon, Toulouse and Bordeaux is cancelled.
The European Commission, which drafts EU laws, estimates that inefficiencies in the way Europe's air traffic is managed add 42km (26 miles) to the average flight.
It wants to centralise air-traffic controls, rather than leave each member state to monitor its own skies. The commission says this could triple the region's airspace capacity, cut costs and reduce delays.
'Violation of sovereignty'
Under the plan, the many national air traffic control systems would be merged into nine Functional Airspace Blocks (FABs), with a new European network manager being given authority for route planning.
But France's main controllers' union, the USAC-CGT, says the plans are "a direct attack on the public service nature of this sector" and a step toward privatisation.
It says the changes constitute a "violation of national sovereignty" and will have an negative impact on working conditions.
In April, the European Transport Workers' Federation (ETF), which is backing the action, criticised what it said was a "never ending process of liberalisation, deregulation and cost cutting" in the air traffic management industry.
Controllers are "suffering from a performance scheme dominated by a never-ending cost reduction and in which safety is not considered to be the first priority", said ETF political secretary Francois Ballestero.
Airports already most affected by the strike include Paris Roissy-Charles de Gaulle, Paris-Orly and Paris-Beauvais,.
Marseille airport, in the south, said on its website that it was less affected than others, with a third of flights cancelled.
More than 70 flights were scrapped out of Nice airport.
Britain's Easyjet said it was cancelling 10-11 daily flights to Toulouse on both Tuesday and Wednesday.
It advised people hoping to fly to contact their airline.
Preparations for the maiden flight of Europe's newest passenger jet, the Airbus A350, have also been affected by the strike.
The French unions' counterparts in some other European countries are set to join in the strikes on Wednesday or hold other industrial action. They are those in Austria, Belgium, Bulgaria, France, Hungary, Italy, Portugal, Spain and Slovakia.
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