Thousands of cab drivers are protesting against Uber and other car-sharing services in major European cities on Wednesday.

The rallies that spread across Berlin, Paris, Madrid and London caused major traffic jams and blocked roads. The drivers' main complaint: Uber doesn't play by the same rules they do by failing to comply with local licensing regulations, not paying taxes and, bottom line, being an unfair competitor.

The protests underscore how Europe's traditional taxi industry has has been shaken up by the introduction of the U.S. startup. Cab drivers often have to pay for costly licensing such as medallions, while Uber drivers usually don't have to, putting traditional cabbies at a disadvantage.

"Everyone should play by the same rules," Richard Leipold, the chairman of the Berlin Taxi Association, told the New York Times. "You can’t have competition between someone who pays all their taxes and someone who doesn’t."

Meanwhile, Uber argues it is simply creating healthy competition in a stagnant market. The company responded to the protests in the UK capital by opening its platform to London's traditional black cabs with the new UberTAXI app, which will allow Londoners to hire a taxi directly through their phones. Until now, you could only hire private drivers through Uber, not traditional, licensed tai drivers.

"In Paris, the number of taxis hasn’t changed since the 1950s," said Pierre-Dimitry Gore-Coty, Uber’s regional general manager for Northern Europe. "The strikes are an attempt to desperately fight against competition in the market."

Taxi drivers in Madrid and Barcelona organized a strike against the startup, even though Uber has not officially launched in the Spanish cities. But many drivers there are using the app to do business anyway, operating in a legal gray area, as the local El Pais reported.

Thousands of taxi drivers just marched down Madrid's Castellana as part of their "war against @Uber" and similar apps pic.twitter.com/PQAqaR53M2 — Nick Leiber (@nickleiber) June 11, 2014

In London, the protests caused major traffic jams.

The startup, which has been valued at $17 billion and now operates in 100 cities in 36 countries, has faced its share of hurdles in its European expansion. Brussels barred Uber from operating there; in Paris, taxi unions won a legal battle in early 2014, forcing Uber and other cab-hailing apps to wait 15 minutes to pick up a passenger after a request.

The ruling was later overturned, but protests in Paris have been ongoing for months, sometimes turning violent. Uber was barred in Berlin, too, but after the decision was appealed, the app continues to operate there in the meantime.