Hundreds of taxi drivers brought part of central London to a standstill in protest at rival service Uber - a mobile phone app.
Cabbies gathered at Trafalgar Square at 14:00 BST for the hour-long protest.
The drivers are angry about what they regard as a lack of regulation of the use of Uber.
The app works out the cost of journeys and cab drivers say it is the same as using a taxi meter, which only black cabs are legally entitled to use.
Areas around Parliament Square and Trafalgar Square were gridlocked During the protest roads were gridlocked around Parliament Square, Whitehall and Trafalgar Square.
Despite the protest Uber said it had seen the number of people downloading its app increase by 850% compared to last Wednesday.
In a statement it said: "The results are clear: London wants Uber in a big way.
"Unsurprisingly, the LTDA (London Taxi Driver Association), which is stuck in the dark ages, is intent on holding London to ransom and causing significant economic impact to Londoners today."
The Metropolitan Police had imposed conditions on protesters after they failed to meet officers to discuss their plans.
Ch Supt Pippa Mills said: "We have attempted to contact those who we believed to be responsible for organising this event...but they have either ignored our approach or been misleading.
Approach roads to Trafalgar Square were brought to a standstill "As a consequence and in order to prevent serious disruption to the life of the community we have been left with no option but to impose conditions on the demonstration."
BBC London's transport correspondent Tom Edwards said he saw Scotland Yard issue a Section 14 to someone at the start of the protest which could have resulted in an arrest.
Steve Garelick of the GMB union said: "I can categorically refute the claim that police have contacted me about this protest."
Black cabs were refused access to Trafalgar Square BBC London 94.9's Anna O'Neill said Haymarket was brought to a standstill and at one point cabs were prevented from accessing Trafalgar Square.
Those in the square were heard chanting "Boris, Boris, Boris, out, out, out", while others beeped horns and held placards.
Black cab driver Bernie Doyle, 68, said: "If Paris, Milan and Berlin don't accept it why should we? I've been driving 42 years and I'm not about to see my trade go down the pan."
Driver Steve said: "We're here protesting against TfL (Transport for London) handing licences out. We're highly regulated, but the others aren't."
Stewart Rose said Uber was "the beginning of the end" for cabbies Stewart Rose, who has been been driving since 1971, said: "It's the beginning of the end for us. If Uber can do it what's going to stop other companies from getting this app?
"Why did they award them the licence before it went to court, they've done it the wrong way round."
Organisations including the Rail and Maritime Transport union, London Cab Drivers Club and Licensed Taxi Drivers Association (LTDA) were represented at the protest.
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.
Taxis started to move away from 15:00 BST In a tweet, the LTDA said: "Do not give up, keep coming , we r winning, if diverted return by any route however circuitous ! United we stand! RT"
The San Francisco-based app launched in 2009 and operates in more than 70 cities across 37 countries.
While unions and taxi driver groups have expressed concern that unlicensed drivers are being contacted using the new app, with no checks on whether they are legitimate, Uber has said "every driver meets all local regulations", having been subjected to insurance and background checks.
TfL is seeking a High Court ruling on whether the use of such an app is legal.
James Potter