A British police officer was shot dead by a young man in south London early morning Friday, the city's Metropolitan Police has said.

The incident occurred at Croydon Custody Centre at around 2:15 a.m. local time (0115 GMT) Friday, the Metropolitan Police said in a statement, adding that a 23-year-old man has been detained.

The police officer was treated at the scene and subsequently died of his injuries at hospital. The suspect was also taken to hospital with a gunshot wound and remains in a critical condition, the Metropolitan Police said.

There were no police firearms discharged during the incident, while the incident has been referred to the Independent Office for Police Conduct, which will "lead an independent investigation," it added.

"This is a truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances," Metropolitan Police Commissioner Cressida Dick said.

The investigation is currently in the early stages, she said, adding that police are working to establish the circumstances surrounding the incident and will provide further updates as they arrive.