(CNN) -- London's taxi drivers are famed for cramming their brains with an encyclopedic knowledge of the city's backstreets that allows them to swerve around any traffic jam.
But on Wednesday many were stuck in gridlock of their own making as they joined other cabbies across Europe staging protests against Uber, a mobile phone app they say threatens their livelihoods.
Uber customers can use their phones to flag down private cabs then meter the charges based on the length of journey.
Drivers of London's black taxis say that shouldn't be allowed.
They say they've earned a monopoly on picking up passengers on the fly and billing by meter through years of study to memorize the city's tangled street map.
To make their point, hundreds of drivers took their black cabs into central London for a horn-tooting drive around Trafalgar Square and past the UK's Houses of Parliament, bringing traffic to a standstill.
Police, who wanted the protest limited to a strict one-hour timeframe, threatened drivers with fines and arrest.
"The long and the short of it is, allowing [a company like Uber] to come in and say they're a tech company, that they don't want to be licensed ... is not fair," said Steve Garelick, a union official representing licensed London taxi drivers.
Several European cities saw similar protests.
In Madrid many drivers stayed home for the day with Spanish media reporting an all-out strike.
In France there were reports of taxis blockading roads in Paris and other cities.
Demonstrations were also staged in Germany and Italy.
In London, transport officials condemned the protest which one report said would cost the city £125 million ($210 million).
Garrett Emmerson, a spokesman for Transport for London, said it was a "pointless disruption for Londoners over a legal issue."
Emmerson said his organization had called on the UK's High Court to make a ruling on what constituted a "taximeter."
Uber, valued at $18 billion, has millions of customers and is expanding its operations to 128 cities in 37 countries.
Its founder, Travis Kalanick, told CNN he often sees a backlash in cities where it gains market clout.
"It's really common when we really succeed in a city that the incumbents -- the taxi industry -- are often trying to protect a monopoly that's been granted to them by local officials. They're trying to slow down competition," he said.
The company said it had benefited from Wednesday's protests, registering an 850% leap in app downloads on the previous week.
Do you prefer to use an app to flag down a ride? Let us know below.
CNN Money's Alanna Petroff contributed to this story