Police have moved to try to avoid travel chaos in London during a taxi drivers' protest by imposing conditions on the demonstration.

Thousands of black cab and licensed taxi drivers will block roads for hours today after driving slowly through the capital from Trafalgar Square.

The row has flared over the introduction of a phone app called Uber which allows customers to book and track vehicles.

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.

The Metropolitan Police said that "repeated attempts" to get the organisers of the demonstration to have a constructive discussion had failed.

Chief Superintendent Pippa Mills said: "We have attempted to contact those who we believed to be responsible for organising this event.

"We even put out a letter through our social media channels to encourage organisers to come forward but they have either ignored our approach or been misleading.

"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.

"Breach of these conditions constitutes an offence under Section 14 of the Public Order Act 1986 and may render the individual breaching the conditions liable to arrest."

Police said the demonstration must not start before 2pm and must not end later than 3pm.

Conditions were also set on the maximum number of people allowed to assemble.

"When the area is congested no further persons will be allowed to join the demonstration. It will be the tactical commander of the event that will determine when the area is congested," said police.

Steve Garelick of the GMB union's professional drivers branch, said: "I can categorically refute the claim that police have contacted me about this protest. I am available on the phone and all Pippa Mills has to do is contact me. Why the police have issued this statement baffles me."

Legal action is being taken against Transport for London (TfL) but drivers are taking direct action, starting with today's protest. Motorists have been advised to avoid central London during the protest.

London Mayor Boris Johnson said he understood the concerns of taxi drivers but added that the issue should be resolved in the courts.

He said London's black cab trade was crucial to the fabric of the city, adding: "The hackney carriage is iconic, and it is absolutely right that our cabbies are properly protected.

"Indeed it is enshrined in law that only black cab drivers have the right to be hailed from the street, and that principle, the bedrock on which our black cabs operate, will never change.

"Black cab drivers are the face of London not just for Londoners but for millions of visitors to our city. Their licence is dependent on passing 'the knowledge' - years of hard graft go into learning every street name, and memorising every conceivable route across that city. That knowledge is why people choose to use black cabs.

"There must, however be a place for new technology to work in harmony with the black cab, and we shouldn't unnecessarily restrict new ideas that are of genuine benefit to Londoners.

"Even so the recent emergence of a range of new apps has raised some important questions in relation the operation of the private hire and taxi trades."

Geoffrey Riesel, chief executive of Radio Taxis, said: "As a former taxi driver, I empathise with London's drivers and respect the fact that they are demonstrating.

"I am naturally concerned as well about our customers and how this will affect our ability to service London in the same professional way in which we've served our capital city for the past 60 years."

He went on: "Our regulatory officials need to wake up to what is happening on our streets.

"It is important that TfL listen and pay attention, first and foremost to take action to ensure that the same high standard of regulation - there to protect the public and not the taxi trade - is enforced, especially where there are new entrants riding rough-shod over rules developed to safeguard passengers."

Mick Cash, acting general secretary of the Rail, Maritime and Transport union, said: "It is shocking that taxi drivers protesting in defence of their trade should be threatened with blanket arrest, and RMT is calling on the police to lift this draconian warning and the threat to criminalise the entire licensed taxi industry.

"This protest is about the ripping up of established metering and hire regulations in the name of greed and private profit and the decimation of London's black cabs."