In one incident last week, an Uber driver was allegedly restrained and threatened while attackers wielding brass knuckles and hammers doused his engine in liquid and slashed his tires, English-language Dutch news site NLTimes reports. The Uber driver involved told authorities one of the assailant’s cars was an Amsterdam taxi.

All the victims involved drive for UberPop, Uber’s European service.

The battle between Uber and traditional taxi companies has often been contentious, but these incidents seem to be some of the most violent on record. Uber, which has often entered markets regardless of local regulations, has faced fierce resistant from some area governments and taxi unions, particularly in Europe.

Uber completed a new round of funding in December valuing the company at over $41 billion, making it the most valuable private startup in the world.