Thousands 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 apps such as 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.
The Licensed Taxi Drivers Association (LTDA) also said part of the demonstration was about highlighting the length of training - between four and seven years - that taxi drivers undergo before being licensed.
During the protest, roads were gridlocked around Parliament Square, Whitehall and Trafalgar Square.
Steve McNamara, from the LTDA, said: "We did not set out to cause disruption to Londoners.
"It is nothing to do with technology, some of the taxis have been using apps for years. The difference is all taxis have to operate legally, they [Uber] should have to apply for the same rules as everyone else."
He added that two drivers had been arrested and 10,000 drivers had attended the protest.
But Scotland Yard denied any arrests had been made and Transport for London estimated that about 4,000 drivers had taken part.
Despite the protest, Uber said it had seen the number of people downloading its app increase by 850% compared to last Wednesday.
The Metropolitan Police had imposed strict conditions on protesters, including limiting the protest to an hour, after they failed to meet officers to discuss their plans.
Ch Sup Pippa Mills said, "Those who took part in today's protest complied with the timings set by police.
"However, we would always encourage organisers to meet with us to discuss their plans in advance so that they can achieve their aims around protest and we can ensure that disruption to the broader community is minimised."
Steve Garelick of the GMB union said: "I can categorically refute the claim that police have contacted me about this protest."
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.
Some motorists sat on the pavements as they waited for the protest to end.
David Batist, 28, said although he had received an email alert from TfL warning him about the potential disruption it had been worse that expected.
"I didn't expect this. I would've taken a different route, had I known. It's caused me quite a lot of disruption. I've had to cancel a meeting."
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."
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.
Some in Trafalgar Square were heard chanting "Boris, Boris, Boris, out, out, out", while others beeped horns and held placards.
Uber launched in 2009 and operates in more than 70 cities across 37 countries.
While those opposed to it are concerned unlicensed drivers are being contacted via the app, with no checks on whether they are legitimate, Uber said "every driver meets all local regulations" and is vetted with insurance and background checks.
TfL is seeking a High Court ruling on whether the use of such an app is legal.
The protests in London follow similar demonstrations in Paris, Madrid, Rome, Milan and Berlin.