The man is wanted in connection with an assault on a security guard, who was knocked out in the middle of the morning after 20 protesters forced their way into City Hall during Mayor's Question Time. The protest centred around Boris Johnson and Transport for London's regulation of private car firms, particularly Uber, which has become a major gripe for London's black cab drivers.
Having made their way inside City Hall, the group attempted to enter the public gallery but they were prevented by security staff and police officers, some of whom were assaulted by the group, the Metropolitan Police said.
The injured security guard was taken by ambulance to hospital where he was treated for concussion and released later that day.
The suspect is described as a white man, around 50-years-old, approximately six feet tall "with a partly shaved head and a receding hairline". He was wearing black leather trousers and a black leather motorcycle jacket with a light brown stripe on the arms.