Pakistan's security forces are facing criticism after paramilitary troops were caught on camera apparently shooting dead a teenager at point-blank range.

The footage, broadcast repeatedly on local television, is likely to further undermine faith in the country's powerful security establishment, which is already facing allegations it helped conceal Osama bin Laden.

The video, captured by a cameraman from Pakistan's Awaz television channel, shows a youth, identified as Sarfaraz Shah, arguing with paramilitary rangers in Karachi. The 18-year-old appears to plead for mercy before being shot at close quarters. He then falls to the ground and screams in pain as blood pools beneath his legs.

Zohra Yusuf, head of Pakistan's independent Human Rights Commission, condemned the killing as "another indication of law enforcement personnel becoming increasingly trigger happy."

She said the violence depicted in the video was a trend seen across Pakistan that reflected the impunity of the country's law enforcers.

Pakistan's prime minister, Yousuf Raza Gilani, said an inquiry would be launched and the culprits punished. Six members of the paramilitary Rangers, who are controlled by the interior ministry, have since been arrested.

Major general Aijaz Chaudhry, who commands the force, described the incident as "deplorable". "The Rangers have no authority to kill any unarmed individual and they can fire only in self-defence," he said. "On completion of the inquiry, all those found responsible will be given strict punishment."

The incident is likely to further dent public faith in the government's ability to control its security forces at a time when the US ally is facing questions about how bin Laden could have hidden for so long without the complicity of intelligence officials.

"What we saw on television shows that now there is the law of the jungle in this country and no one is accountable for his action or deeds. This is pathetic," Mohammad Sultan, a retired soldier, told Reuters.

"What we are seeing is visual records of what we have long documented, which is the culture of impunity in the Pakistani law enforcement agencies," said Ali Dayan Hasan, a researcher for Human Rights Watch. "What is becoming clear is that the free for all, the culture of wanton abuse and killing, is becoming untenable in the age of new media and cell phone cameras."

In one media interview, a man identified as Salik Shah, the victim's brother, said: "My brother was a victim of barbarism, brutality and aggression and everyone has seen it. The innocent young man was begging for his life regardless of whether he had done anything wrong. He was asking to be pardoned by the rangers; despite his repeated requests they did not listen to him, they did not arrest him, instead they were adamant about killing him and in the end they did."

Hundreds of people showed up at Shah's funeral a day after his death and denounced the Rangers. Some shouted "Rangers, murderers!" and others carried signs that said "Down with the Karachi Rangers."

The video's broadcast comes a few days after a prominent journalist was tortured to death after reporting claims about al-Qaida. Military intelligence officials have rejected claims they played a role in the killing.

Pakistan's Daily Times newspaper said the military, paramilitary forces, police and intelligence agencies "who confidently violate human rights" should be held accountable for their actions.

"The security and law enforcement forces that do not respect the law themselves are inviting anarchy, which arguably is already under way," it said in an editorial.

Last year, a video emerged of two teenage brothers being beaten to death before being strung up on a metal pole in broad daylight as police personnel looked on.