An ambulance crew on an emergency call to a stroke patient were forced to stop after an object was thrown at their vehicle.
The windscreen was smashed as paramedics responded to a call in Handsworth, Birmingham.
It was shattered prompting an alternative ambulance to be sent to the emergency after it was reported by crew.
West Midland Ambulance Service said that the smashing of the windscreen "delayed the response" to a patient by about 10 minutes.
The incident, which happened in the area's Worlds End Road on Saturday was captured on the vehicle's CCTV and the ambulance service say will be passed to police.
The ambulance, which is based at the Erdington hub, was damaged at around 8.20am.
Emergency services operations delivery director Nathan Hudson, told the BBC it was "difficult to comprehend" why someone would attack an emergency vehicle.
He said: "There is only one thing that an ambulance travelling on blue lights is doing and that is trying to get to a patient in need.
"Given where the damage is, I am just so glad that none of the crew were hurt. It doesn't bear thinking about, what could have happened."
Mr Hudson said the ambulance service would work with police to try and find and prosecute the person responsible.
The attack is the latest to hit ambulance and follows incidents in last year.
The Mirror reported how three crews were targeted leading to two paramedics needing hospital attention.
North East Ambulance Service (NEAS) staff were affected as they tried to treat patients during a weekend of violence
During one incident in Blyth, ambulance workers were forced to shelter in the back of their van with a patient who required treatment, as the assailant allegedly attacked the vehicle with bricks, the NEAS said.
At another incident in South Shields, a patient allegedly became "physically violent" in the back of the ambulance and damaged equipment.
And in South Hetton, Co. Durham a technician was allegedly "kicked and punched" and later required hospital treatment to check for internal injuries, after they became unwell.
