Police failed to recognise a man was suffering from mental illness despite displaying obvious signs, an inquest heard.
Sean Rigg, 40, of Fairmount Road, near Streatham Hill, died while in police custody on August 21, 2008.
He was arrested for public order offences after attacking passers-by in Weir Road, Balham, but collapsed shortly afterwards while being held in police custody.
Mr Rigg was later pronounced dead while in hospital.
Police Constable Richard Glasson, who was working at Brixton Police Station, said police chased Mr Rigg to a paved area in Weir Road, Balham after responding to calls from members of the public.
Southwark Coroner's Court heard three officers put Mr Rigg in the rear stacks position, with his arms behind his back, so they were able to handcuff him.
They also searched his pockets and retrieved a passport, but did not initially identify him as the person in the photograph.
He was then arrested for having a stolen passport and public order offences, before they drove him to Brixton Police Station.
It was only when Mr Rigg began moving his legs up and down the sides of the van that police thought he might have mental health problems.
Coroner Dr Andrew Harris said: "Did it not strike you as odd he was wearing nothing on his top?"
PC Glasson said: "Some people do, as soon as there is a bit of warmth they have their tops off. At that point I was not aware of his mental illness."
The court heard there was no space for Mr Rigg in the police custody suite when they arrived, so he was placed in a caged waiting area at the back of the station.
He kept on trying to sit down on the floor to lie down, but police officers put him back in the standing position.
However within minutes he convulsed forward and collapsed on the floor, appearing to suffer from a fit.
The in-station doctor was called for and officers performed CPR until paramedics arrived to take him to hospital.
CCTV footage, taken from outside the caged area, was played to the jury and an unidentified person could be heard suggesting Mr Rigg was feigning the fit.
PC Glasson said: "We do have to consider all of those options, it is something we have to consider."
During the inquest the coroner read out a police training document which said people with mental health problems could be identified by removing clothing for no reason or not speaking.
PC Glasson said: "He was not speaking to us at that point. It was unusual, didn’t know what the reason it was for at that time. We knew he had been acting aggressively to people."
Another training document said: "Physical stress on the person’s body may result in death. Provision of emergency medical care must take priority over mental health care, medical care must be sought immediately."
The inquest continues.