The drivers of London's famous black cabs brought the centre of the capital to a standstill on Wednesday in protest against Uber and government regulations they say compromise passenger safety.
San Francisco-based Uber, whose investors include Goldman Sachs and Google, has grown rapidly around the world but has faced protests, bans and restrictions in several cities.
On Wednesday, thousands of black taxis lined up in the main streets around the Houses of Parliament and Prime Minister David Cameron's 10 Downing Street residence to choke traffic for around 90 minutes.
The unions said drivers were protesting against the relaxation of rules put in place to restrict who could pick passengers up off the street, rules they said were designed to increase security for passengers.
They say the growth in private hire cars like those using Uber has also impacted congestion and air pollution.
"We feel that Uber exploits its drivers by using their self-employed status to extract maximum income from them which translates to those drivers working long hours," said Jim Kelly at the Unite union.
"The reduction in safety for passengers because of the 'light touch' regime is to be deplored. It is a race-to-the bottom, when in 2016 we want the highest possible standards."
London transport bosses decided in January not to impose a series of strict new rules on private hire cars, including those using apps like Uber.