(CNN) — Hundreds of flights have been canceled and thousands of passengers forced to change plans as French air traffic controllers took industrial action over pay on Friday.

Europe’s biggest airline Ryanair grounded 420 of its flights across Europe as the 24-hour strike affected connections from airports around the continent. Rival budget carrier EasyJet was warning of severe delays and disruption.

Members of France’s Syndicat National des Contrôleurs du Trafic Aérien, or SNCTA, the main union for traffic controllers in the country, began striking from 6 a.m. central European time (00:00 a.m. ET) Friday. Knock-on disruptions are expected to carry on into Monday.

The union said it was calling the action after negotiations stalled with the government over pay rises in line with inflation.

“After several months spent negotiating to get fair and adapted answers, SNCTA… laments the lack of concrete elements and guarantees from public authorities,” it said in a statement.

More delays

French aviation authorities warned up to 50% of flights could be affected by the strike.

French aviation authorities warned up to 50% of flights could be affected by the strike.

Horacio Villalobos/Corbis News/Corbis/Getty Images

Paris Aéroport, which owns and manages 14 civil airports and airfields in the Île-de-France (Paris) area, warned on Thursday to expect “significant” delays and cancellations on arrivals and departures.

Many flights passing over French airspace were also affected. Barcelona Airport’s departure board showed more than 50 flights delayed or canceled after noon Friday. In Germany, Hamburg Airport said 48 out of the day’s 251 flights had been canceled by midday local time and further delays and cancellations were possible.

Earlier in the week, France’s Directorate General for Civil Aviation warned that up to 50% of the country’s flights could be affected and urged passengers to contact airlines and postpone travel.

The SNCTA has said that a strike scheduled for 28 September could go ahead.

Disruption was also expected on Monday as London’s Heathrow airport said some flights would be canceled or delayed to keep airspace silent during events to mark the funeral of Queen Elizabeth II.

CNN’s Arnaud Siad and Peter Taggart contributed to this story.