Mayor's Question Time was shut down after police were called to City Hall today, after a security guard was apparently knocked out in scuffle outside the building.

Black cab drivers were demonstrating outside (and inside) the Southbank venue over ongoing criticism of Transport for London's handling of regulation for private hire car companies, in particular Uber.

They waved banners calling for Johnson to "stand firm" against "Uber's lobbyists", with suggestions that the ride-sharing taxi app puts public safety at risk.

https://twitter.com/ParkerCabbie/status/644081169915637760

https://twitter.com/TheLTDA/status/644081329009762304

But what was a peaceful, albeit rowdy protest, descended into violance after a number of protesters were expelled from the gallery when Johnson dubbed them "luddites".

Chaos at City Hall as chair orders taxi drivers expelled from gallery mid question from @CarolinePidgeon after mayor calls them "Luddites". — Stephen Knight (@StephenKnight1) September 16, 2015

A security guard was reportedly knocked "unconscious" as tensions rose. The London Ambulance Service has confirmed that one person was taken to hospital for minor injuries.

The protest is ongoing outside of City Hall however. Police have advised motorists to avoid the area, with the protest causing congestion in nearby roads.

You scared boris we are waiting!!! pic.twitter.com/K7hdCkZz7d — MRSKINS99 (@MRSKINS99) September 16, 2015

There is currently a protest causing congestion nr City Hall and Tower Bridge. Police are in attendance. Please avoid the area for now ^KJR — MPS Southwark – Central South Command (@MPSSouthwark) September 16, 2015

The iconic London taxis have ground traffic to a halt from Tower Bridge and along Tooley Street towards London Bridge.

The protest is apparently causing queues from Tower Bridge to East Smithfield and as far south as Borough High Street.

Demo- Q's on Tooley St over London Bridge. Traffic also slow on Borough High St. S/bnd q's across Tower Bridge to East Smithfield. — TfL Traffic News (@TfLTrafficNews) September 16, 2015

Pictures posted on social media showed what appeared to be hundreds of protesters turning out in support of black cabs this morning, despite the rain.

https://twitter.com/Supportblktaxis/status/644096971884613633

https://twitter.com/Supportblktaxis/status/644091834927587328

https://twitter.com/Supportblktaxis/status/644097107524194304

Supporters took no small pleasure from the fact the protest had blocked at least one Uber driver.

https://twitter.com/Supportblktaxis/status/644092707850317824