Gunmen shot dead nine policemen and prison staff as they slept in Lahore on Thursday, the second attack on security forces in the country's political heartland since Islamabad reopened a NATO supply corridor.
The killings, and a similar assault on an army camp on Monday, raise fears of a fresh wave of violence in Punjab, the country's most populous province that will be vital in upcoming elections, and which for the past year had seen a lull in Taliban attacks.
IG Punjab Habibur Rehman said the attackers came on three motorcycles and one car, armed with Kalashnikov rifles and hand grenades.
They then stormed the building in the densely populated area of Ichra, where around 30 police and prison staff were sleeping early Thursday.
One survivor told AFP from his hospital bed that staff jumped frantically onto the roof tops of neighbouring houses to escape the hail of bullets.
"About 15 of us were sleeping on the roof and some were offering prayers when gunfire started downstairs. Some of my colleagues who went down to see what was happening were killed or wounded," Mohammad Rizwan Shah, 23, told AFP.
He said he works in the prison of Peshawar and came to Lahore six weeks ago for a training course due to end on July 28.
"I jumped over to the house next door to save my life and fractured my arm. Others too jumped walls and into neighbouring houses," he said.
Police said nine other people were wounded, and that the victims were shot as they slept on simple mats now soaked with blood.
Local resident, Mohammad Siddiq, 43, told AFP he saw gunmen fleeing on motorcycles and a blood-drenched body lying in the street at around 5:30 am.
Pakistan's main umbrella Taliban faction claimed responsibility.
Spokesman Ehsanullah Ehsan told AFP in a telephone call that the attack was carried out because the officers being trained "are used in operations against us".
"We want to prove that no place is beyond our reach," he added.
Police blamed the "same gang" who killed seven security personnel at an army camp in Gujrat, 150 kilometres (100 miles) southeast of Islamabad, on Monday.
A senior security official said Monday's attackers were "highly likely" to have belonged to a banned organisation in league with the Taliban.
President Asif Ali Zardari and Prime Minister Raja Parvez Ashraf have strongly denounced the incident of firing on police personnel in Lahore.
In their messages‚ they said such incidents can not deter the government resolve to wipe out the menace of extremism and terrorism.
Senior Advisor to the Prime Minister Rehman Malik has also condemned the firing incident in Lahore.