A blind 61-year-old was left "terrified" after being hit with a 50,000 volt taser by police when they mistook his white cane for a samurai sword.
Stroke victim Colin Farmer collapsed to the ground in shock after he was hit in the back with the gun while walking down the street in Chorley, Lancashire.
"There was shouting loudly and aggressively just as though I was going to be mugged," he said afterwards, adding: "I dropped to the floor with my cane and I was shouting all the time 'I'm blind! I'm blind!'"
Police had received reports of a man walking through the town armed with the deadly martial arts weapon and raced to the scene to hunt down the offender.
Meanwhile Mr Farmer, who has suffered two strokes and walks at a "snail's pace", was on his way to the pub to meet friends and did not realise anything was afoot.
"Lancashire Constabulary deeply regrets what has happened. We have clearly put this man through a traumatic experience and we are extremely sorry," read a statement from the force.
"We have launched an urgent investigation to understand what lessons can be learned and the matter has also been referred to the Independent Police Complaints Commission."
Mr Farmer is reportedly considering legal action.