One German pilot told news reporters: “We will see what today brings. It could be that there could be more strikes.”
Today’s 24-hour walkout involves staff in Germany, Sweden, Ireland, Belgium and the Netherlands.
Last night, a court in the Netherlands said pilots from that country could take part in the industrial action.
About 50,000 passengers are understood to have been told of cancellations on 400 flights.
Ryanair says 85% of its scheduled flights will operate as normal on Saturday and the majority of customers affected have been given places on other Ryanair flights.
The airline said: “Ryanair took every step to minimise the disruption and we notified our customers as early as possible, advising them of their free move, refund or reroute options.
“The majority of customers have already been accommodated on another Ryanair flight.
“We want to again apologise to customers affected by this unnecessary disruption and we ask the striking unions to continue negotiations instead of calling any more unjustified strikes.”
