A "terrified" blind man was hit with a 50,000 volt Taser gun after police mistook his white stick for a samurai sword.
Stroke victim Colin Farmer, 61, collapsed to the ground in shock after he was hit in the back with the gun while walking down the street in Chorley, Lancashire.
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 for a drink and was unaware of the reports.
He had no idea a police officer had mistaken his thin white stick for a machete and ordered him to stop before pulling the trigger on the Taser gun.
Mr Farmer cried out, "I'm blind! I'm blind!" while collapsed on the floor but was still handcuffed by the officer from Lancashire Police.
"The whole thing was like being trapped in a nightmare," he told the Chorley Guardian.
Mr Farmer said he heard shouting on the street but did not know what it was about and thought he was about to be "attacked by some hooligans".
He was then struck by the Taser and fell to the ground, dropping his white stick on the floor before a policeman handcuffed him.
He was taken to Chorley Hospital for treatment and was later released following the incident at about 5.45pm last Friday.
Chief Superintendent Stuart Williams, from Lancashire Police, said: "We received a number of reports that a man was walking through Chorley armed with a samurai sword and patrols were sent to look for the man.
"One of the officers believed he had located the offender. Despite asking the man to stop, he failed to do so and the officer discharged his Taser.
"It then became apparent this man was not the person we were looking for and officers attended to him straight away.
"He was taken Chorley Hospital by officers who stayed while he was checked over by medics. They then took him to meet his friends in Chorley at his request.
"Lancashire Constabulary deeply regrets what has happened. We have clearly put this man through a traumatic experience and we are extremely sorry.
"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 against the force.