Roger Silvester, 30, was described yesterday as being in a "critical but stable" condition in hospital.
He was detained by eight Metropolitan Police officers after they were called to a report of a disturbance outside a block of flats in Tottenham, north London.
Police say the officers in three cars arrived at just before 10pm on Monday. "A naked man was reportedly banging on the door of a neighbour in an aggressive and vociferous manner," said a Scotland Yard spokeswoman.
"Prior to police attending the man had been rolling about on the floor in the garden area in an agitated state," she added.
She went on: "Officers attended and restrained the 30-year-old man - he was handcuffed for his own safety and that of others. Neither batons nor CS spray were used and at no time was he under arrest. The officers' main concern was for the well-being of the man."
A friend of the family was reported as claiming that there was an altercation between the officers and Mr Silvester, who is believed to have a history of mental illness, during which he suffered injuries to his head and hands.
A police spokeswoman said there were no records of a fight, but that all allegations would be examined during the forthcoming inquiry, which is being overseen by the Police Complaints Authority.
After handcuffing the man the officers contacted the local authority and took Mr Silvester, who was detained under the Mental Health Act, in a police van to St Ann's Hospital, north London to be assessed by a mental health practitioner.
The police spokeswoman said: "While at the hospital the man suffered respiratory failure and was resuscitated."
He was taken by ambulance to North Middlesex Hospital where he was treated and later transferred to the Whittington Hospital, where he is breathing on a ventilator.
His family have been at his bedside since he was admitted, unconscious, on Monday night.
A spokesman for the family said they were very upset and asked for their privacy to be respected. "The family ask for everyone to remain calm.
"The family want to receive urgent assurances from the Police Complaints Authority and area health authority that every minute of what happened on 11 and 12 January is accounted for."Reuse content