Metropolitan Police Commissioner Sir Bernard Hogan-Howe has praised the courage of frontline officers after a constable was left fighting for his life when he was hit by a speeding car.
Pc Andrew Duncan, 47, was critically injured when he was run over by a black Volkswagen Golf that he had tried to stop for speeding just before 1am in Sutton, south London.
A 25-year-old man has been arrested on suspicion of causing serious injury by dangerous driving after he went to a police station in south London.
Sir Bernard, who is Britain's most senior police officer, said: "An officer injured on duty is always upsetting for all of us in the Met and our best wishes are with him at this time. We will, of course, ensure that he, his family and his colleagues are being fully supported and a thorough investigation into the incident has already begun.
"Whilst serious injuries on duty are thankfully rare, it is a reminder of the courage of our frontline staff when carrying out their role to keep London safe."
Pc Duncan, who was hit on the A217 Reigate Avenue near to the Rosehill junction while he was on patrol with another constable, remains at a hospital in south London.
Commander Dave Martin said: "Andy is a hard-working and committed police officer. He is extremely popular with everyone he comes into contact with. The thoughts of the Metropolitan Police Service are with Andy and his family at this distressing time, we are all wishing for his recovery."
The car that hit the officer, who is based at the south west traffic garage in Merton, south London, was found abandoned nearby.
Steve Williams, chairman of the Police Federation of England and Wales, said: "It's another example of the inherent dangers of policing. It's only 12 months since the deaths of Fiona Bone and Nicola Hughes and it's another stark reality of how dangerous the job of policing can be. Our thoughts are very much with his colleagues and family."
Officers are appealing for witnesses to come forward. Anyone with any information can call police on 020 8941 9011.