LONDON — Ryanair cabin crews in several European countries went on strike on Wednesday, the start of a planned two days of industrial action that has forced the low-cost carrier to cancel hundreds of flights, affecting around 50,000 passengers.

The strike — the biggest that Ryanair has had to grapple with — highlights the shifting fortunes of the Irish carrier, an icon of Europe’s cut-price flight boom. Following the brash and aggressive character of its chief executive, Michael O’Leary, Ryanair made its name by cutting costs to the bone and vociferously opposing union membership.

But Ryanair now faces multiple challenges. Profits are down sharply, and fares are expected to remain low. Rivals from Eastern Europe are competing for customers. And increasing numbers of strikes are forcing the airline to face up to the fact that it is no longer a young disrupter.

In the latest sign of Ryanair’s problems, the strike on Wednesday involved cabin crews in Belgium, Italy, Portugal and Spain and was set to continue through Thursday. The employees say that Ryanair has ignored grievances about low pay, contracts that do not offer sufficient employment rights and a punishing workplace culture.