A woman suffered serious injuries after being run over by a police car.
The accident happened while officers were responding to a 999 call last night about a man allegedly making threats with a knife.
Avon and Somerset Constabulary said the woman was left with "possible life-threatening head injuries" following the collision on the outskirts of Bristol city centre.
A force spokesman said: "The police vehicle was on an emergency blue light run responding to reports of a man with a knife making threats in the Stokes Croft area.
"At about 10pm the police vehicle was involved in a collision with the woman at the junction of Gloucester Road and Ninetree Hill.
"The woman, who suffered possible life-threatening head injuries, has been taken to Southmead Hospital."
Bristol police said its professional standards department had been made aware of the incident - as has the Independent Police Complaints Commission watchdog.
Officers added that a man had been arrested on suspicion of assault and a knife has been seized.
"We are keen to hear from anyone who may have been in the area at the time who may have information to help our investigation," added the spokesman.
Anyone able to help is asked to contact the force's collision investigation unit on the force number 101.