A man has died after he was reportedly chased by police and then "tackled to the ground" by officers.
The police watchdog has launched an investigation following the death in Bedford on Wednesday afternoon.
Police performed first aid on the victim and the ambulance service attended but he died at the scene in Midland Road.
A witness told the Bedfordshire News website: "The police tackled him to the ground and he seemed to smash his head off the floor.
"Then they tried to perform CPR."
The Independent Police Complaints Commission has appealed for information from any witnesses.
Bedfordshire Police said the incident happened at about 12.35pm.
A spokeswoman said: "Officers gave first aid at the scene and the ambulance service also attended, however the man has sadly died.
"The IPCC will be informed of the incident, as is procedure when a person dies who has been in recent contact with the police."
The IPCC tweeted: "We have begun an independent investigation following the death of a man in Bedford this afternoon, following contact with @bedspolice."
Witnesses are asked to contact the IPCC via witness@ipcc.gsi.gov.uk or 0800 096 9070.
