Uber drivers are staging strikes in several UK cities in a protest over pay and working conditions.
"Hundreds" of drivers are expected to switch off their apps from 1pm, making themselves unavailable for work, union organisers said.
Protests are also being held outside the ride-hailing app's offices in London, Nottingham and Birmingham.
Uber defended its pay record, saying it had rolled out numerous benefits for employees in the last few months.
The rallies are being organised by The United Private Hire Drivers (UPHD) branch of the Independent Workers Union of Great Britain (IWGB).
It is demanding a fare increase to £2 a mile, a reduction in commission paid by drivers to Uber, and an end to what it calls "unfair" driver deactivations.
The IWGB is also calling on Uber to honour an employment tribunal verdict in 2016 which rejected the ride-hailing app's claim that drivers are self-employed and not entitled to minimum wage or paid holidays.
'Respect the app picket line'
James Farrar, head of the UPHD branch, said "hundreds" are expected to join the strikes.
"If you look at social media feeds its viral at the moment. In the history of our union, I've never seen anything quite like it."
Images of protests are being posted on social media. Additional police supervision was called to Uber's London headquarters after several demonstrators tried to storm the building.
Deputy Labour leader Tom Watson was among those expressing support for the strike on Twitter. John McDonnell, the party's shadow chancellor, also called on people to "respect the app picket line."
An Uber spokesperson said the company stood by its pay record, adding that it had introduced sickness, injury, maternity and paternity protections "over the last few months."
The firm did not say how the strikes would affect its service.
Tuesday's walkout comes after strikes by employees of UberEats, McDonalds, Wetherspoons and TGI Fridays last week over pay disputes.
In a separate protest on the same day, 50 UberEats couriers, Uber drivers and supporters temporarily occupied the lobby of Uber's London headquarters.
Uber was recently valued at $72bn, making it one of the most valuable privately held firms in the world.
But the company's expansion into food delivery and bike sharing have eaten into recent earnings.
In August, the firm reported a 51% annual increase in income from its taxi app business, but adjusted losses in the previous three months were up by almost a third on the previous quarter.
Dara Khosrowshahi, Uber's chief executive, said the company was "continuing to grow at an impressive rate for a business of our scale."