The police watchdog has opened an investigation following the death of a man the day after he was arrested by Metropolitan police officers in south-east London.

The 26-year-old was detained on suspicion of being in possession of a knife near Elephant and Castle on Friday, but died on Saturday after swallowing a substance while in police custody at Walworth station, the Met said.

The Independent Office for Police Conduct (IOPC) has opened an investigation into the incident after it was referred to the watchdog by the Met. A postmortem examination will also be held.

Police said they arrested a man at about 11.30am on Friday after a lock knife was discovered by officers during a car search on Brandon Street, Southwark. An ambulance was called shortly after the detainee arrived in custody as officers believed he had swallowed something.

He arrived at hospital at about 12.15pm, but died on Saturday morning after his condition deteriorated.

IOPC regional director Sarah Green said: “My thoughts are with the young man’s family and friends at this most difficult time, as well as all those affected by this tragic incident.

“We were made aware of the man’s arrest, and that he had been taken to hospital, on the afternoon of Friday 30 March and we sent our investigators to the custody suite and to post-incident procedures. They have been working since that time to gather all the available evidence at this early stage of our investigation. We will be offering the man’s family regular updates on the progress of our investigation.”