Thousands of black cabs and licensed taxis converged on Trafalgar Square this afternoon in protest over the mobile app Uber, bringing gridlock to large swathes of central London.

Up to an estimated 12,000 drivers took part in the “go-slow” demonstration, which began at 2pm, intent on causing disruption to the capital’s roads.

Drivers’ unions said that the rise of Uber is leading people to contact unlicensed drivers without any checks on whether they are legitimate.

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The app lets customers track vehicles and book them with a couple of taps of a smartphone, but critics say its system for working out prices is tantamount to a taxi meter – which only black cabs can legally use in London.

In central London this afternoon, taxi drivers beeped their horns in unison as demonstrators held placards. Protesters, angry at Transport for London for seemingly siding with Uber in the row, expressed their frustrations with the Mayor Boris Johnson, chanting “Boris, Boris, Boris, out, out, out.”

Shape Created with Sketch. In pictures: European taxi drivers in protest against Uber app Show all 8 left Created with Sketch. right Created with Sketch. Shape Created with Sketch. In pictures: European taxi drivers in protest against Uber app 1/8 London Taxi drivers in protest at Victoria Street in London 2/8 London Black cab and licensed taxi drivers protest at Trafalgar Square in London. Unions and groups representing taxi drivers are warning that the move is leading to unlicensed drivers being contacted via the new technology, with no checks on whether they are legitimate 3/8 London Black cab and licensed taxi drivers protest at Trafalgar Square, London over the introduction of a phone app called Uber which allows customers to book and track vehicles 4/8 London Traffic gets worst as taxi drivers protest at Trafalgar Square in London 5/8 Barcelona A taxi proceeds demonstrators holding a banner during a strike action in protest of unliscensed taxi-type-services in Barcelona 6/8 Barcelona Taxi drivers hold a banner during a strike action in protest of unliscensed taxi-type-services in Barcelona. Banner reads, "Out with illegal apps" 7/8 Madrid Taxi drivers carry a banner during a strike action in protest of unliscensed taxi-type-services in central Madrid 8/8 Madrid Taxi drivers in London, Paris, Madrid and other European capitals plan to bring chaos to the streets in protest against unlicensed mobile car-hailing services such as Uber which have shaken up the industry 1/8 London Taxi drivers in protest at Victoria Street in London 2/8 London Black cab and licensed taxi drivers protest at Trafalgar Square in London. Unions and groups representing taxi drivers are warning that the move is leading to unlicensed drivers being contacted via the new technology, with no checks on whether they are legitimate 3/8 London Black cab and licensed taxi drivers protest at Trafalgar Square, London over the introduction of a phone app called Uber which allows customers to book and track vehicles 4/8 London Traffic gets worst as taxi drivers protest at Trafalgar Square in London 5/8 Barcelona A taxi proceeds demonstrators holding a banner during a strike action in protest of unliscensed taxi-type-services in Barcelona 6/8 Barcelona Taxi drivers hold a banner during a strike action in protest of unliscensed taxi-type-services in Barcelona. Banner reads, "Out with illegal apps" 7/8 Madrid Taxi drivers carry a banner during a strike action in protest of unliscensed taxi-type-services in central Madrid 8/8 Madrid Taxi drivers in London, Paris, Madrid and other European capitals plan to bring chaos to the streets in protest against unlicensed mobile car-hailing services such as Uber which have shaken up the industry

A police helicopter hovered above the scene while uniformed officers patrolled the area.

Scotland Yard had earlier moved to avoid travel chaos in London by imposing conditions on the demonstration, including limiting it to an hour.

Organisations including the Rail and Maritime Transport union (RMT), London Cab Drivers Club (LCDC) and Licensed Taxi Drivers Association (LTDA) were all represented at the protest in Trafalgar Square.

Derrick Hoare, of the LTDA, said they wanted to highlight the length of training - between four and seven years - taxi drivers undergo before being licensed.

The RMT's Ian Beetlestone said he hoped the demonstration would send a message that Uber is “operating in a grey area”.

He said: “We are not objecting to competition. We have had competition for years from minicabs but we haven't caused gridlock over it.

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“We have to jump through hoops to be regulated and we don't feel people involved in these new apps are being subjected to the same regulations.”

Similar taxi driver protests took place today in Madrid, Milan, Berlin and Paris. An Uber spokesperson said the company was “committed to keeping people moving across all the impacted European cities”, adding: “Wednesday will be treated like any other busy period for us.”

Uber now operates in more than 100 cities in 30 countries and last week was valued at $18.2 billion, a fivefold increase in the space of a year. If the valuation is to be believed, an app launched five years ago is now worth more than the global car hire firm Hertz, which was founded in 1918.

“In fact, today we're seeing an 850 per cent increase in sign-ups compared to last Wednesday. The results are clear: London wants Uber in a big way.

“Unsurprisingly, the LTDA, which is stuck in the dark ages, is intent on holding London to ransom and causing significant economic impact to Londoners today, estimated to be £125 million.

“We join Transport for London and the Metropolitan Police in calling on the London Taxi Drivers Association and others to call off this unnecessary and irresponsible strike.

“We passed TfL's most stringent and comprehensive audit of a Private Hire Vehicle operator to date, passing with flying colours.

“We are proud to be to be in London, we are proud to serve London, and we are here to stay. Uber on London.”