A boy has been taken to hospital with serious injuries after being struck by an ambulance in Bath.
London Road is still closed on the westbound side at Snow Hill because of the incident.
Police sealed off the road, and were even stopping pedestrians going past the scene of the incident at one point.
The boy - whose age has not been confirmed but is believed to be ten or 11 - has been taken to the Royal United Hospital with a serious head injury.
Bosses from South Western Ambulance Service - whose vehicle was en route to an emergency at the time - have travelled to Bath to investigate what happened.
The service has been unable to confirm suggestions that the ambulance - thought to be heading for Swainswick - was on the westbound carriageway at the time.
The crew of the ambulance treated the boy at the scene of the incident and then took him immediately onto the RUH.
Police investigating what happened allowed them to leave the scene immediately because of the seriousness of the boy's injuries.
The incident caused widespread traffic disruption.
Buses were forced to take diversions away from the scene.
Elsewhere, the traffic chaos was intensified by a jack-knifed lorry at Limpley Stoke, affecting the A36 around Brassknocker Hill, and an accident on the A46 at Swainswick.
B&NES Council said London Road westbound was likely to be closed for up to three hours. The eastbound side has been reopened.
The incident is close to the Esso petrol station.
B&NES said: "Signage is being provided for the closure by the council. This will result in large volumes of traffic going onto other routes. The council will be monitoring traffic flows and making adjustments to signal timings in order to keep traffic moving as much as possible."
A statement from the ambulance service said: "At 4.01pm this afternoon, an ambulance crew responding to a 999 emergency reported to our clinical hub that they had struck a child near the junction with London Road and Snow Hill in Bath. They reported the child had suffered a significant head injury.
"The patient was placed in the ambulance and taken by the crew straight to Royal United Hospital where he is now being treated. Due to the serious condition of the patient, police gave permission for the ambulance to leave the scene to take him to hospital.
"Clearly our primary concern is for the patient and his family – senior executives from South Western Ambulance Service have travelled to Bath and will offer to meet with the family. We are also ensuring the crew involved are fully supported
"We are also working closely with the police to ensure the circumstance surrounding this serious incident are fully investigated."
