Britain's most senior police officer has led the tributes to a long-serving colleague who was killed in a hit-and-run while trying to halt a speeding car.
Metropolitan Police Commissioner Sir Bernard Hogan-Howe described Pc Andrew Duncan as "courageous" after he was seriously injured when he was struck by a vehicle in Sutton, south London, in the early hours of Friday.
The married father-of-two, 47, died in hospital this morning. A man is currently being held in custody on suspicion of murder.
Speaking from New Scotland Yard, Sir Bernard said: "Andy was a hard-working and courageous policeman. He served the community as both a police officer and as a Scouting volunteer.
"The night he was injured, he was doing a job that he loved and we all had every right to expect he would return safely to his home and family - tragically that was not the case."
Pc Duncan, a member of the south-west traffic unit for nearly 10 years, was working on a speed enforcement operation with a colleague when they noticed a black Volkswagen Golf driving at speed along Reigate Avenue toward Rosehill, Sutton.
He attempted to stop the car by signalling for the driver to pull over but he was struck by the vehicle. The car was later found abandoned nearby.
Mayor of London Boris Johnson was among those to pay tribute to the constable.
He said: "Pc Duncan was an officer of vast experience who'd dedicated his working life to the Metropolitan Police Service.
"His death is a tragedy and a reminder of the courage and sacrifice shown daily by the men and women who keep London safe.
"My thoughts and those of every Londoner are with Andrew's wife and children at this time."
Shadow Home Secretary Yvette Cooper said: "I want to send my deepest condolences to the family of Pc Duncan and to his Metropolitan Police colleagues who I know will be feeling this deeply.
"Commissioner Hogan-Howe has rightly drawn attention to Pc Duncan's courage.
"He was doing a job each and every one of us relies upon to keep us safe, tragically losing his life in the service of others.
"Police officers up and down the country face risks every day. We owe them a debt of immense gratitude."
Pc Duncan joined the Metropolitan Police in 1990 and was originally posted to Battersea.
He transferred to the south west territorial support group six years later before moving to Hammersmith and Fulham Borough after seven years' service.
Sir Bernard, who is Britain's most senior police officer, added: "I was so sad to hear that Pc Duncan died this morning.
"On behalf of the Metropolitan Police family, I would like to offer my sincere condolences to his wife and children, parents, family and friends.
"We will continue to professionally and objectively investigate this incident under the oversight of Her Majesty's Coroner.
"I would urge anyone with information to contact us."
A 25-year-old man originally arrested on suspicion of causing serious injury while dangerous driving has now been arrested on suspicion of murder.
Scotland Yard confirmed a woman has also been arrested on suspicion of perverting the course of justice.
She remains in custody, police said.
Anyone with information is asked to call police on 020 8941 9011.