Ryanair cabin crew in Portugal began a walkout on Wednesday as counterparts in Spain announced 10 days of strikes.
A separate stoppage by Ireland-based pilots was blocked by court action, but the airline lost a similar case in England where action can now go ahead.
Europe's biggest budget carrier is trying to avoid the travel chaos it suffered last year when it was hit by a wave of strikes. It has since agreed to recognise trade unions for the first time, but has not been able to reach agreement with employee groups in all the countries where it has bases.
In Portugal, a five-day strike starting Wednesday was called by Portugal’s SNPVAC union in a dispute over pay and conditions.
Ryanair said flights from its Portuguese bases might be affected by delays or schedule changes but reported no cancellations at Lisbon, Porto and Faro airports.
Belgium’s CNE and ACVPULS trade unions told members not to comply with a Ryanair request to work on flights affected by the Portuguese strike.
In Spain, unions said cabin crew would strike at 13 bases in September after no agreement was reached in several hours of talks over job cuts.
USO and SICTPLA gave notice of stoppages on September 1, 2, 6, 8, 13, 15, 20, 22, 27 and 29 in protest at plans to close staff bases in Gran Canaria, Tenerife South and Girona.
Jairo Gonzalo, secretary of USO for Ryanair, said in a statement the airline had shown an “attitude of mockery” and there was “not the slightest hope” of reaching an agreement.
In Ireland, a court granted Ryanair’s application for an injunction, preventing some Ireland-based pilots at the airline from taking strike action on Thursday and Friday.
The airline said the ruling was a “huge relief” to passengers at the end of the school holidays and called on a “minority of very well-paid pilots” to end their dispute.
Bernard Harbor, of the Fórsa union, said outside court: "There will not now be any industrial action this week. We are going to consider the ruling in detail and consult again with our legal team, once we have the ruling in writing."
Ryanair managed to quell disputes last year by reaching deals with many unions in Europe on pay and allowances, but it has yet to move beyond recognition agreements with others and further angered unions by telling staff that it had 900 more pilots and crew than it required.
In July, it reported post-tax profit down 21% year on year, at 243 million euros, driven by competition in Germany, Brexit uncertainty and a question mark over the return to service of Boeing's grounded 737 MAX.
Separately, ground workers in Barcelona for Iberia announced a strike on August 24, 25, 30 and 31.