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A British police officer has been shot dead in the early hours of Friday by a man detained in police custody at Croydon Custody Centre in Windmill Lane, in South London.

The officer was treated at the police station by officers and paramedics but died later in hospital.

The London Metropolitan Police said a murder investigation has been launched into the death of the officer and a 23-year-old man was detained by officers at the scene and taken to hospital with a gunshot wound. He is now in a critical condition.

"This is a truly shocking incident in which one of our colleagues has lost his life in the most tragic circumstances," London Metropolitan Police commissioner Cressida Dick said. "My heart goes out to his family, direct colleagues and friends."

A murder investigation is now underway and the Independent Office for Police Conduct (IOPC) will launch a separate independent investigation into the incident.