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.
Stand firm Boris! pic.twitter.com/FgH5SjvS7E— The LTDA (@TheLTDA) September 16, 2015
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 (@MPSSouthwark) September 16, 2015
The iconic London taxis have ground traffic to a halt from Tower Bridge and along Tooley Street towards London Bridge.
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.