Ryanair planes were grounded in six European countries yesterday, with staff walkouts disrupting more than 40,000 passengers' plans.
Europe's biggest low-cost carrier has faced multiple strikes since it bowed to pressure to recognise trade unions for the first time last December, with staff stepping up pressure in talks over pay and conditions.
The impact on the airline's bookings has forced it to consider cutting its short-term growth plans.
Cabin crews in Germany, Belgium, Portugal, the Netherlands, Spain and Italy, as well as pilots in Germany, began a 24-hour walkout as airspace opened yesterday.
"They don't care about us. We are just numbers," said Juan Fernandez, a former Ryanair employee who joined protesters at Brussels Airport.
"We are lemons to be squeezed and thrown away."
Strikers at the airport held up placards with the message: "Ryanair must change."
The company, which cancelled 150 flights due to an "unnecessary strike" by a "tiny minority of cabin crew", cancelled up to 100 more on Thursday after German pilots said they would join the protest.
Ryanair said yesterday that more than 2,150 of its 2,400 flights would operate.
The action in some of its largest markets will be the second-biggest one-day strike at the airline.
Around 55,000 customers were affected last month when pilots in five European countries walked out at the height of the summer holiday season.
Rival easyJet has forecast a higher annual profit than expected with robust demand and cancellations at Ryanair.
The dispute chiefly centres on the airline's practice of employing many of its staff under Irish law, which unions say causes members difficulties, stopping them from receiving local social security benefits.
A spokesman for Ryanair said the company had already written to the unions in Belgium and all other EU countries, offering to take on board local law and social security taxes.
In Frankfurt, German pilots' union Vereinigung Cockpit warned of more action.
"We have no other solution," said a spokesman.