Pakistan's Prime Minister, Yousuf Raza Gilani, has ordered an inquiry into the killing - on camera - of a young man by paramilitaries.
The Rangers say the young man was caught trying to rob someone. His family denies this.
An inquiry is already under way into the killing of five unarmed Chechens last month by the security forces.
Pleads for lifeThe video from Karachi, which has been broadcast widely on Pakistani television, is disturbing to watch.
It shows a young man in a black t-shirt being dragged by his hair in a public park by a man in plain clothes.
He is pushed towards a group of Sindh Rangers, a paramilitary force, who are in uniform and armed. The young man, who does not appear to be armed, pleads for his life as one of the Rangers points a gun at his neck.
A little later, a Ranger shoots him twice at close range, hitting him in the thigh. The young man is seen writhing on the ground, bleeding heavily and begging for help.
The paramilitaries remain close to the injured man but do nothing to help him. He died from his injuries.
The Rangers have since said that the young man had been caught trying to rob someone. His family denies this.
His brother, Salik Shah, said Sarfaraz had been the victim of an extrajudicial killing.
"My innocent brother has been killed brutally by the Rangers, " he said. "What harm has he inflicted on anybody?
"His crime was that he was just strolling in the public park because there was no power in the house."
The incident has sparked an outcry with some politicians calling for the Rangers involved to be prosecuted.It is not clear who shot the video which has been broadcast repeatedly on Pakistani television. Some reports suggest that the cameraman has now gone into hiding.
Ali Dayan Hasan, the Pakistani researcher for Human Rights Watch, said the killing "provides chilling evidence of the abusive, trigger-happy practices of Pakistan's military, paramilitary and civilian agencies".
It comes as a separate inquiry is under way in neighbouring Balochistan Province into the killing of five Chechens by members of the paramilitary.
Five civilians, including a heavily pregnant woman, were shot dead at close range in the western city of Quetta, despite apparently trying to surrender.
Officials initially said that the five family members were armed and were suicide bombers - but this was later found to be untrue.
