A young girl who was on foot and three adults in a car were killed when a tipper truck collided with vehicles and pedestrians on a hill in Bath, Avon and Somerset Police said.
Police are investigating the incident which happened in Lansdown Lane in Upper Weston in Bath, Somerset, just after 4pm today.
Weston All Saints Primary School, which is situated close to the scene of the incident, will be closed tomorrow. A spokesman for Bath and North East Somerset Council said: "Due to the tragic accident on Lansdown Lane this evening, the police have asked that Weston All Saints Primary be closed tomorrow. For further updates on opening the information will be available at the schools closure page."
Chris Lucas, a Liberal Democrat campaigner, took a picture of two air ambulance helicopters landing on a playing field shortly after the crash. He said: "My children were coming home and as I opened the front door my neighbour said 'Oh my God there has been a crash, I'm going to call the emergency services'. We didn't know at that point how bad it was.
"I didn't see the accident itself, I saw two helicopters landing in the field but there were several buzzing around. One thing that is really is concerning is that road is on the school pick-up run. Weston All Saints Primary School is right next to it. All fatalities are awful, but I am praying there are no children involved. It is a very tight community here and one of my children goes to that school. My thoughts go out to everyone involved."
Early witness accounts suggest the driver was trying to avert an accident, an Avon and Somerset Police spokesman said.
"Four people have died at the scene and there are several people injured," the force spokesman said. "Casualties are being taken to the Royal United Hospital in Bath and Southmead Hospital in Bristol. We're working with South Western Ambulance Service and Avon Fire and Rescue Service at the scene. The area is closed off to allow for investigative work to take place."
South Western Ambulance Service NHS Foundation Trust's spokesman said: "We can confirm that we have taken one patient by air ambulance to Southmead Hospital in Bristol. Three patients have been taken to the Royal United Hospital in Bath. There were two air ambulances at the scene, five ambulances, three paramedics in rapid response vehicles and a BASICS doctor."
A spokeswoman for South Western Ambulance Service NHS Foundation Trust confirmed the patient taken to Southmead Hospital in Bristol by air ambulance was in a "critical" condition. The conditions of three other patients who were taken to the Royal United Hospital in Bath by land ambulance are not yet known.
Further casualties with less serious injuries are being assessed and treated by paramedics at a nearby school. "They are walking wounded," the ambulance spokeswoman added.