Police officers have changed their social media profile pictures in tribute to their colleague shot dead in Croydon.
The officer was shot in a police station in the early hours of Friday morning and later died in hospital, police said.
Politicians, police chiefs and other officers have condemned the attack and offered their condolences to the family and friends of the man who died.
And some officers have updated their display picture to show a blue line against a black background, or posted the image online, as a mark of respect for their colleague.
Stuart Hutson said he was one of the officers that responded first to the incident.
He added in a tweet: “This morning my team and I responded to the worst possible radio transmission from custody, words and scenes I shall never forget.
“The unimaginable happened to our police family. We have lost not only a good skipper but also a real gentleman. One of the best. RIP brother”.
Officers in the Metropolitan Police’s firearms unit said they were “desperately sad” about the death of their colleague.
They said in a tweet: “As one @metpoliceuk family we are desperately sad to have lost a friend and colleague last night.
“Our thoughts are with all affected friends and family. We exist to protect London from armed criminality: this incident therefore weighs on us profoundly. We will never forget”.
A senior police office who tweets under the name the Met Skipper said he was “struggling to find the words” to discuss his colleague’s death.
He said: “My thoughts are with my blue family.”
Prime Minister Boris Johnson, London Mayor Sadiq Khan, Met Police commissioner Cressida Dick and many other politicians have also offered their condolences after the “devastating” killing.
Scotland Yard said in a statement: “At approximately 0215 on Friday, September 25 the officer was shot by a man who was being detained at Croydon Custody Centre in Windmill Lane.
“Officers and paramedics treated him at the scene and he was taken to hospital by London Ambulance Service. Very tragically he subsequently died at hospital.
“We are in the process of informing all of the officer’s family and are supporting them with specialist officers.
“A 23-year-old man was detained by officers at the scene. He was also taken to hospital with a gunshot wound and remains in a critical condition.
“No police firearms were discharged during the incident.”