East Midlands ambulance workers were the victims of almost 350 violent incidents in the last year.
Workers feel increasingly vulnerable when sent out to deal with emergency situations, a union spokesman said.
The Association of Professional Ambulance Personnel is calling for stab protection vests to be routinely issued to all staff.
The union said there is a worrying rise in the number of assaults involving knives across the country.
Phil Morris from East Midlands Ambulance Service said: "This situation unfortunately reflects the national picture.
"It is extremely frustrating for staff to react at speed to somebody who needs some help only for that person or bystanders to turn on them."
There are more than 110,000 attacks against NHS staff in the UK each year, with a higher proportion of those committed against ambulance crews than doctors and nurses, the union said.
Stab vests have been rejected by ambulance trusts in the past as they are seen as being too cumbersome and give off a military appearance.
London Ambulance staff were given vests in 2002 and Essex Ambulance Service introduced them in May this year.