A Metropolitan Police officer who was shot dead at a south London custody suite was a "long-serving sergeant", Met Commissioner Cressida Dick has said.
The victim died in hospital after the gunman, who was being detained, opened fire at Croydon custody centre in south London during the early hours of Friday.
The 23-year-old murder suspect, who is believed to have shot himself, is in a critical condition in hospital.
Scotland Yard said no police firearms were fired during the incident at around 2.15am.
A murder probe has been launched and investigators from the Independent Office for Police Conduct (IOPC) watchdog are on the scene to establish how the gun got into the custody suite.
My deepest condolences go to the family, friends and colleagues of the police officer who was killed in Croydon last night.
We owe a huge debt to those who risk their own lives to keep us safe.
— Boris Johnson (@BorisJohnson) September 25, 2020
Met Commissioner Dame Cressida Dick said: "This morning we learnt of the shocking death of a much-loved colleague, a long-serving sergeant in the Metropolitan Police who was working last night in our Croydon custody suite.
"I have visited and spoken to our officer's partner together with other colleagues. We are giving her the best support we can."
She added: "Early indications are that the suspect shot himself. This has not yet been established as fact. The man remains in a critical condition in hospital.
"I understand that there is considerable interest in the identity of the officer but we have not yet been able to inform all of his close family."
Leroy Logan, a former Met superintendent, said there were questions to be answered around the circumstances which led to the shooting.
"How did that person come to be in the station, whether it's in the yard or the building itself, and be able to produce a weapon, whether it's on them at the time?" he told BBC News.
"It depends on the calibre of the weapon, because obviously if it's a small weapon and it can be easily in that person's clothing, then obviously it brings another question on how thoroughly that person was searched, if at all.
"Those are the things the department for professional standards will look at and the IOPC as well as the investigating officers who will have to look at this thing thoroughly."
The officer is thought to be the first to be killed in a shooting in the line of duty since Pcs Fiona Bone, 32, and Nicola Hughes, 23, in September 2012.
They were murdered by Dale Cregan in a gun and grenade attack while responding to a report of a burglary in Greater Manchester.
The Met sergeant is the 17th from the force to be killed by a firearm since the end of the Second World War, according to the National Police Memorial roll of honour.
Unarmed Pc Keith Palmer, who was stabbed in March 2017 by terrorist Khalid Masood during the Westminster Bridge attack, was the last Met officer to be killed in the line of duty.
The roll of honour includes Pc Andrew Harper, who died when he was caught in a tow rope and dragged along country lanes after trying to stop quad bike thieves in Berkshire in August 2019.
The Thames Valley Police officer's three teenage killers were cleared of murder but convicted of manslaughter after an Old Bailey trial.
His widow Lissie Harper, who is campaigning for a change to the law which would see all those convicted of killing emergency workers receive a life sentence, said: "This is devastating news.
"No person should go to work never to return. No human being should be stripped of their life in a barbaric act of crime.
"Another hero has been taken from us in unwarranted violence."
Prime Minister Boris Johnson was among those who reacted to the news on Friday morning, tweeting: "My deepest condolences go to the family, friends and colleagues of the police officer who was killed in Croydon last night.
"We owe a huge debt to those who risk their own lives to keep us safe."
Home Secretary Priti Patel said: "I'm deeply shocked and saddened by the tragic killing of the officer in Croydon overnight.
"All our thoughts are with the officer's family, friends and colleagues across the Metropolitan Police force, but also policing family across the country.
Commissioner Cressida Dick: "The Met is a family. Policing is a family, within London and across the UK, and today we are all mourning a great loss.
"This terrible incident underlines once again how police officers face danger every day in their work to protect the public." pic.twitter.com/pvIWSyFXo6
— Metropolitan Police (@metpoliceuk) September 25, 2020
"This is a sad day for our country as once again we see the tragic killing of a police officer in the line of duty as they're trying to protect us and keep us safe."
Mayor of London Sadiq Khan added: "I am devastated by the news a Metropolitan Police officer has lost his life.
"I was informed of this tragic incident by the Commissioner this morning, and my heart goes out to the family of this brave officer, who has paid the ultimate price for helping to keep Londoners safe."
Forensic officers in white suits were seen entering the police station on Friday.
Devastated by this news. My heart goes out to the family of this brave officer, who has paid the ultimate price for helping to keep Londoners safe.
Tragic incidents like this are terrible reminders of the dangers our police officers face every single day. https://t.co/LQ7PHNxwJt
— Sadiq Khan (@SadiqKhan) September 25, 2020
Local resident Wilhemina Dew, 45, said: "It makes me feel scared."
She added: "Whatever the reason, whatever the tensions shooting the police is unacceptable because we all need to feel safe."
Reverend Catherine Tucker, of the Holy Church Croydon parish, said: "The action taken against the police is really unacceptable but I also feel sorry for the perpetrator.
"Sadly, I am not surprised there has been a shooting in Croydon."
She added: "There are tensions between the police and young people particularly in relation to stop and search and the way the police relate to the community."