The incident is said to have taken place at about 3 pm.
She was said to have been pelting stones at passing vehicles since the early hours of Monday and even broke the windshields of four of them.
An eyewitness who spoke to the media said that the woman pulled the victim down from the motorbike and smashed his head with a huge stone.
“The man died before passersby arrived at the scene”, the eyewitness added.
Police officers, who later rushed to the scene, handcuffed the woman and took her away to prevent an angry mob from lynching her on the spot.
She was later sent to the Nima Police Station as the police commenced investigations into the incident.
The video of the moment the mentally challenged woman stoned the unidentified young man in broad daylight at the full glare of passers-by has popped up.
In the video, the deranged woman picked up a stone and hit the young man several times until he was left unconscious with blood oozing from his body till he drew his last breath.
There have been speculations that the mentally deranged woman is the mother of the deceased which some dispute.
Moments later after the killing incident, a car with registration number GS3499-17 was also involved in an accident at the same stretch, but the driver escaped unscathed.
He was however taken to the hospital for treatment.