The Metropolitan Police said the officer died in hospital after being shot by the man at Croydon custody centre in south London during the early hours of Friday.
A 23-year-old man detained at the scene was also taken to hospital with a gunshot wound and remains in a critical condition, police said.
The force said no police firearms were discharged during the incident.
When a colleague dies in the line of duty the shockwaves and sadness reverberates throughout the Met and our communities
Met Commissioner Cressida Dick said: “This is a truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances. My heart goes out to his family, direct colleagues and friends.
“We are currently supporting his family and also have a dedicated team providing support to the officers and those in the custody centre who witnessed the shooting.
“When a colleague dies in the line of duty the shockwaves and sadness reverberates throughout the Met and our communities. Policing is a family, within London and nationally, and we will all deeply mourn our colleague.
“We are in the early stages of the investigation and are still working to establish the circumstances surrounding the incident and we will provide further updates when we have them.”
My statement on the tragic incident in Croydon. pic.twitter.com/5FdDhCNCPt
— Priti Patel (@pritipatel) September 25, 2020
Home Secretary Priti Patel said: “I am deeply shocked and saddened to learn that a Metropolitan Police Officer has been shot and killed in the line of duty.
“My thoughts today are with his family, friends and policing colleagues in London and across the country.
“This morning I spoke to Metropolitan Police Commissioner Cressida Dick to express my condolences and to offer whatever support is needed as this tragic event is investigated.
“This is a sad day for our country and another terrible reminder of how our police officers put themselves in danger each and every day to keep the rest of us safe.”
The incident has been referred to Independent Office for Police Conduct watchdog which will lead an independent investigation. The Met continues to investigate the officer’s murder.
The officer’s family are being told.
