Thousands of flights were cancelled in France and Europe on Thursday as a strike by French air traffic controllers over pay caused stoppages not seen in decades.

Airlines cut more than half of their normal flight schedules at Paris's two main airports Orly and Roissy Charles-de-Gaulle, with many flights in the southern city of Marseille also grounded.

Short and medium-haul flights were the worst affected, while long-haul services faced the least disruption.

Budget airline Ryanair reported 300 cancellations Thursday, and easyJet and Transavia each 200.

A total of some 2,300 flights arriving at or departing from French airports were forecast for Thursday, against 5,200 the day before, civil aviation authority DGAC told AFP.

Across Europe, some 2,000 more flights were cancelled and another 1,000 forced to change routes to avoid French airspace, the Airlines for Europe trade body said.

Cancellation numbers were at their highest "in 20 years", said Augustin de Romanet, CEO of ADP, the company operating the capital's airports.

On Wednesday, De Romanet had warned that "several tens of millions of euros (dollars)" were at stake for airlines and airports "every strike day".

Air traffic association IATA accused the French controllers of "blackmail" and of making exorbitant demands.

A decision by the biggest air traffic controller union SNCTA on Wednesday to withdraw their strike call after a deal with management came too late to alleviate the mayhem significantly as the DGAC had already ordered airlines to cancel most of their flights.

Three smaller unions, meanwhile, maintained their strike call.

Delays for flights authorised to operate were moderate, however, DGAC said, with many only around 30 minutes late.

French holidaymakers were particularly hard hit by the disruption, with many of the country's schools on spring break.

"There's not much to be done," said one passenger giving only her first name, Chloe.

She said she and a fellow passenger had flown into Paris from Shanghai without being able to connect to their home city of Clermont-Ferrand, 420 kilometres (260 miles) to the south.

"My husband got in his car at 4:00 am in Clermont to come and pick us up," she said.

Several airlines told customers they could change their reservations for free or get a refund for their tickets.

The unions' demands are a response to a planned overhaul of French air-traffic control systems.

The strike calls followed an initial breakdown of talks, raising concerns over the risk of renewed action during the Olympic Games in Paris from late July, when millions of visitors are expected in the capital.