The London office owned by taxi app firm Hailo has been vandalised as tensions mount in the capital over alternative cab services.
Black-cab drivers are angry that Hailo is opening up its service to private hire vehicles.
The word "Scabs" was scrawled on the wall of its London office and police were called after a fight broke out.
Discontent is growing across Europe between licensed taxi drivers and alternative private hire vehicles.
The app firm, co-founded by three London cabbies, had been exclusively for black cabs and allows people to hail a cab through their smartphone.
In an open letter to London taxi drivers, chairman Ron Zeghibe explained why the firm had applied for a private hire licence.
"There is no point burying our heads in the sand - people want a choice," he said.
"When we started, it was a straight fight between taxis and private hire. Now, it's not so simple. These are tough times that call for tough decisions - and that means doing what's right, not what's popular."
He was referring to services such as Uber, a San Francisco-based start-up that describes itself as a "pick-up" service.
It connects those needing a ride with a background-checked private driver, and takes a cut - typically 20% - of the fee.
It now operates in more than 100 cities across 30 countries.
Steve McNamara, head of the London Taxi Drivers' Association said that "feelings were running high".
Of the incident at the Hailo office he said: "Things turned a little bit nasty, punches were thrown and the police were called."
Beyond that, he said, drivers were deleting the app and "queuing up" to have the Hailo stickers removed from their cabs.
Licensed taxi drivers around Europe are getting increasingly frustrated by the proliferation of new services such as Uber, which they say are not subject to the same regulation and licensing regime as them.
In the Italian city of Milan, taxis have been sitting idle for the past five days in protest at what they say is "unfair competition" from Uber. In Paris, the government is considering banning Uber drivers from using GPS-enabled apps and in Spain the National Taxi Federation has called for it to be banned saying it is putting 100,000 jobs at risk.
In London too, black-cab drivers are planning a protest on 11 June over the way Transport for London has handled Uber's arrival in the capital.
"Our beef is not with Uber but with the regulator which is not enforcing the law and kowtowing in the face of Uber's money," said Mr McNamara.
"The reason for the complete collapse of a normally strong and vigilant regulator can only be put down to the fact that TfL are intimidated by the money, power and influence being brought to bear by the enormous presence of Uber's backers Google and Goldman Sachs."
He said that the protest, likely to cause gridlock in central London had "united the taxi and minicab trades in London".
At the heart of the dispute is why Uber is classified as a minicab service when it uses a smartphone app to calculate fares. Black-cab drivers argue that the app is akin to a taximeter and it is illegal for such vehicles to be fitted with these.