An ambulance crew had to lock themselves in their vehicle after a gang of men smashed the driver's window and kicked the doors.
The ambulance is not in use following the attack
Around eight attackers tried to break into the ambulance in pursuit of a man who had sought refuge in the vehicle.
The paramedic and medical technician were shaken but unhurt following the incident at Rhyl in Denbighshire.
Three local men were arrested but later released without charge. The ambulance is now off the road for repairs.
The incident happened on Wednesday after the ambulance crew responded to a 999 call.
As the crew were helping their patient into the ambulance a young man jumped in through the back door.
A spokesman for the Welsh Ambulance Service said it appeared that the man was trying to escape from approximately eight other men gathered outside.
One of the attackers smashed the glass in the driver's cab and tried to climb into the ambulance.
The crew, who do not want to speak to the media about the ordeal, told him to get out whilst trying to shield their patient and the man seeking refuge.
North Wales regional ambulance officer Dafydd Jones-Morris said the attack was "despicable".
"This mindless act of a few individuals will affect many more people in the area with the loss of a vital emergency ambulance that will now be unavailable to us whilst forensic tests and later repairs take place," he added.