A long-serving police officer has been shot dead at Croydon Custody Centre in south London.
The male sergeant was shot in the chest before the suspect turned the firearm on himself, sources have told the BBC.
The man had been brought to the custody suite in a police vehicle and the shooting happened during questioning about Covid-19, the BBC was told.
“This morning we learnt of the shocking death of a much loved colleague,” said Met Police Commissioner Cressida Dick.
The victim, who has not been named, is thought to have been a few weeks away from retirement and was described as “one of a kind” by a colleague.
BBC Home Affairs correspondent Danny Shaw said it was believed the suspect was known to counter-terrorism police having been on their radar in the past, though the Met Police has not officially confirmed that.
The man, who is thought to have been detained for possessing ammunition, has since been arrested on suspicion of murder.
No police firearms were discharged during the incident, which happened at about 02:15 BST at the Windmill Road centre.
The Met chief said the policing family was “deeply shocked and very sad” following the death of the officer.
“I have visited and spoken to our officer’s partner together with other colleagues and we are of course giving her the best support we can,” said the Met chief.
“My heartfelt condolences go to her, to their family, to his colleagues and his close friends.
“A murder investigation is under way and officers are working at several crime scenes to secure evidence and to establish the facts of what happened.
“Early indications are that the suspect shot himself, this has not yet of course been established as a fact.”
Det Insp Richard Berns described his colleague as “hard working and an inspiration to all who knew him”.
“It was a privilege to have worked with him and known him over so many years,” he said.
“He was was one of a kind and will be deeply missed. Rest in peace my friend.”
Community police officer Jacqueline Kufuor was among those laying flowers outside the custody centre in tribute to her colleague.
She described the officer as “a lovely guy” and “the nicest man I have ever met”.
An investigation by the Independent Office for Police Conduct (IOPC) is under way and will have several strands, our correspondent Danny Shaw added.
“It’s likely to focus on the circumstances of the man’s arrest – which officers were deployed during the operation; whether and how the suspect was searched; and if he was put into handcuffs,” he said.
“The IOPC will also need to establish what happened at the police station and whether appropriate measures were put in place when the suspect was taken out of the police van.”
Earlier, Prime Minister Boris Johnson said: “We owe a huge debt to those who risk their own lives to keep us safe.”
In a post on social media he also said: “My deepest condolences go to the family, friends and colleagues of the police officer who was killed in Croydon last night.”
By Dominic Casciani, BBC Home Affairs correspondent
This appalling incident in Croydon appears to be absolutely unique – an officer shot by a man who was already inside a police facility – and the shock felt today underlines how rare it is for police officers in the UK to be killed by a suspect in the line of duty, relative to other nations.
The Metropolitan Police officer shot dead in Croydon is the 17th from the force to have been killed by a firearm since the Second World War.
But since the beginning of the 20th Century only 73 police officers have been shot and killed by criminals in the UK, excluding all deaths in Northern Ireland.
The majority of those deaths – more than 50 – have occurred since 1945.
Police officers in other parts of the world are often puzzled why British constables are not routinely armed. But the fact is that there are very few criminal guns in circulation – and the culture of policing has never seen it as acceptable to be universally armed.
However, Tasers are increasingly a common sight in the UK – and a massive survey of police officers recently found three-quarters would carry one of the less-than-lethal devices on the frontline, if given the choice.
Yogarajah Emmanuel, 43, who runs a shop opposite the custody suite, said he woke up at 02:30 BST to the sound of sirens.
“I looked out of my window and could see three ambulances,” said Mr Emmanuel.
“There was noise and all of a sudden one ambulance from inside the car park came out and sped off.
“This morning I heard it was a police officer and just felt so sad. They are all very good people and wave and say hello when they come to my shop.”
At the scene: Thomas Mackintosh, BBC London
Met Police officers have been standing outside the custody suite doing something they dread – mourning one of their own.
Several bouquets of flowers have been placed outside the building in memory of the sergeant who was shot.
Residents who live opposite have shared their shock and disbelief that such a tragic murder has happened so close to home.
Forensic teams have been spotted in the nearby yard gathering evidence, while other officers can be seen inside the suite carrying on with their duties.
‘Sick to their stomachs’
Ken Marsh, chairman of the Met Police Federation, said news of the shooting was “utterly devastating”.
“Officers across London are in shock and sick to their stomachs at the nature of his death,” he said.
“Sadly, on very rare occasions officers make the ultimate sacrifice whilst fulfilling their role. When that happens we will ensure their bravery and sacrifice is never forgotten.”
The incident has been referred to the Independent Office for Police Conduct (IOPC), which will lead an independent investigation.