An investigation has been launched by the Independent Police Complaints Commission after a man was taken to hospital in a serious condition shortly after he was arrested.
Police were called to a report of a disturbance at the junction of High Street and Paris Street in Exeter city centre shortly before 11am on Wednesday.
As a result, a 32-year-old man was restrained and arrested on suspicion of a public order offence.
At around 11.10am, the man, who has not been named, was taken by police van to the custody unit at Heavitree Road Police Station.
The IPCC said concerns developed for the man's health while he was in custody and became unresponsive. He was taken to the Royal Devon and Exeter Hospital at 12.15pm, where he is in a serious condition with head injuries.
IPCC Commissioner Amerdeep Somal said an investigation was under way.
"This incident unfolded in a busy part of Exeter city centre and it is important for our independent investigation that anyone who witnessed events or saw or spoke to the man before police attended to come forward," she said.
"This was a very serious incident involving several police officers and a man is now in a serious condition in hospital.
"We are reviewing relevant CCTV footage of the city centre and are anxious for the public's help in determining exactly what happened."
Ms Somal added that CCTV of the custody suite was also being analysed and preliminary accounts had been taken from any officers involved.
He said the IPCC had contacted the man's family to inform them of their involvement and explain how the investigation will progress.
Witnesses can contact the IPCC in confidence on 0800 096 9077 or email firstname.lastname@example.org