Hundreds of European flights have been hit by air traffic control strikes

(CNN) – Hundreds of flights were canceled and thousands of passengers were forced to change plans by French air traffic controllers over pay on Friday.

Europe’s biggest airline Ryanair grounded 420 of its flights across Europe as the 24-hour strike disrupted connections at airports across the continent. Rival budget carrier EasyJet reported severe delays and disruptions.

France’s Syndicat National des Contrôleurs du Trafic Aérien, or SNCTA, the country’s main traffic controller union, went on strike Friday at 6 a.m. CET (12 a.m. ET). The offensive outages are expected to last through Monday.

The union said it was calling the action after negotiations with the government over an inflation-adjusted pay rise broke down.

“After spending several months negotiating to obtain fair and appropriate responses, SNCTA … deplores the lack of specific elements and guarantees from public authorities,” it said in a statement.

More delays

French aviation authorities warned that 50% of flights could be affected by strikes.

Horacio Villalobos/Corbis News/Corbis/Getty Images

Paris Aéroport, which owns and operates 14 airports and civil airports in the Île-de-France (Paris) area, warned on Thursday that it expects “significant” delays and cancellations in arrivals and departures.

Many flights passing through French airspace were also affected. Barcelona airport’s departures board had delayed or canceled more than 50 flights since midday on Friday. In Germany, Hamburg Airport said 48 of the day’s 251 flights had been canceled by noon local time and more delays and cancellations were possible.

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

SNCTA has said that the strike planned for September 28 can go ahead.

Disruption was also expected as London’s Heathrow Airport said on Monday that some flights would be canceled or delayed in order to quiet the airspace during events to mark the funeral of Queen Elizabeth II.

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