At 10.45am on Tuesday a patrol car was called to the hotel in question because of an assault on two individuals, according to a police source. When they arrived, they found four people beating up one of the two men who had complained and causing damage to his vehicle the source added.

Three of the four individuals accused of being the attackers have been identified, police said, adding that two of these were taxi drivers.

One of the two injured men, who asked that his name not be used, and who has a company that works with Uber, said that he and one of his drivers were attacked "with punches and kicks" in the "back and head" in front of the Hotel Sheraton and that the violence only ended when they sought refuge inside the hotel.

Lusa contacted the Sheraton but was unable to get a comment.

In a statement to Lusa, Uber said that it "vehemently condemns all acts of violence."

Florêncio Almeida, the president of Portugal's main tax association, ANTRAL, condemned the violence but stressed that Uber "is not authorised to work in Portugal" and that "the law should be applied" and "the vehicles [using the app] should have been seized."

Portugal's government "must do something," Almeida said. "I fear that the situation will slip out of control."

ANTRAL has secured a court injunction against Uber to stop it from operating in Portugal but the company has appealed against the move.

The latest incident came on a day when a man was found guilty by a Lisbon court of last year causing criminal damage to a car whose driver was using the Uber app to pick up clients.