Emergency crews are trying to recover a car from a canal in Bristol in what police call "a serious incident".
Detectives said a search operation is under way, but no one has yet been recovered from the vehicle.
Ambulances and fire crews are also at the scene in Feeder Road.
Eyewitnesses described the car as "sinking like a stone" after it crashed through a barrier and into the water.
Recording and rehearsal studio boss Steve Mallison, 44, was helping bands unload gear into the nearby RS Studios when the crash happened.
He said: "The car came down the road near Short Street and failed to stop when it got to the turning by Feeder Road.
"It went straight into the railings and literally went straight to the bottom.
"Nobody has been pulled from the car. If anyone has survived that I would be very surprised.
"A middle aged couple in the street nearby were really traumatised by what they had seen.
"Loads of emergency services then raced down to the scene - I've never seen so many police cars and ambulances in quick succession.
"There's been a few cars which have gone in the river in the past 10 or 15 years we've been here, but none as bad as this.
"Whatever has happened it's looking very serious. There's 40 vehicles in total and two river dive teams."
Avon and Somerset Constabulary said they were called to the incident in the Feeder Canal at around 6pm on Sunday.
A force spokesman said: "A car, driving from the direction of Short Street, failed to stop on Feeder Road and drove into the canal.
"We are currently on the scene with the fire and ambulance service. At this stage, a search and rescue operation is under way but no-one has been recovered from the vehicle at present.
"We have put in place several road closures in the local area including Short Street and Albert Crescent at the roundabout with Albert Road, Marsh Lane, Avon Street and Feeder Road. We would ask all motorists to avoid this area at the present time."
Anyone who may have witnessed the incident or seen the vehicle beforehand, is asked to contact 101 now and quote log number 741.