German pilot representatives had said this week they were joining the strike action with a 24-hour walkout, ending at 2:59 am Saturday, because they want pay and work conditions comparable to those at Ryanair's competitors.
The company has pointed to recent pay increases and invitations to meet for negotiations. Ryanair urged the unions "to continue negotiations instead of calling any more unjustified strikes".
Ryanair built its low-cost business model without unions, but said last year it would recognize them. Labour representatives are seeking collective-bargaining agreements in the different countries.
In the Netherlands, the carrier was using non-striking pilots to keep its service running for passengers.
In a tweet yesterday night, the company said that "there will be no cancellations" as a result of the Dutch union's strike.
In Sweden, some 40 Ryanair pilots walked out to demand a collective labour agreement.
At Ryanair's main Belgian hub, the Charleroi airport south of Brussels, a few dozen striking pilots gathered in the main terminal behind a banner marked "Ryanair on strike, Ryanair must change".
The Belgian Cockpit Association's Alain Vanalderweireldt said the strike in Belgium "is the conclusion of six months of discussions between Ryanair and the union representatives that has led to nowhere concretely".
While the union wants to apply Belgian labour laws to employees, Ryanair is still applying Irish laws, he said.
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