A passer-by who called 999 prior to a man's death in custody said it was obvious he had mental health problems.
Sean Rigg, 40, of Fairmount Road near Streatham Hill, who suffered from schizophrenia, died while in police custody on August 21, 2008.
He was arrested on suspicion of public order offences and attacking a police officer in Weir Road, Balham, after witness Liam Jung dialled emergency services.
But he was taken ill at Brixton police station and was pronounced dead in hospital later that night.
Southwark Coroner's court heard on Thursday Mr Jung felt compelled to call the police after he saw Mr Rigg karate chop a man close to Kings Avenue, in Balham.
He said: "We stopped at the junction and I saw a man, he was only wearing bottoms. He had nothing on his torso at all.
"He started to do karate moves when I looked at him. I felt I could not just let that go and the gentleman had to be stopped."
Mr Jung's 999 call was played to the jury in which he describes Mr Rigg as "insane" and "starting on everybody he sees."
The person handling the call said: "He must have mental health issues."
Sherry White, who witnessed the police chase, said she saw Mr Rigg running towards her car and thought it was strange he was topless.
Earlier jurors heard 999 calls made by staff in Mr Rigg's community hostel requesting help were rejected by civilian handlers.
The court was told he was known to police in the area, who were aware he had mental health problems and had helped take him to hospital in the past.
The inquest continues.