Gunmen on Thursday shot dead eight Pakistani police and prison staff, and wounded nine others after storming a building in the eastern city of Lahore where they were sleeping, police said.
It was the second attack in three days on security personnel in the province of Punjab, raising fears of a fresh wave of violence in the political heartland of Pakistan away from the northwest where a Taliban insurgency is based.
The attackers arrived on motorbikes and targeted a building in the densely populated area of Ichra, where up to 35 police and prison staff were living, mostly officers from the troubled northwest who were in Lahore for training.
"The gunmen came early in the morning, entered the building and opened fire," Lahore police chief Aslam Tareen told AFP.
"Eight were martyred and nine others are wounded."
The gunmen fled and there was no immediate claim of responsibility.
Pakistan is battling an Islamist insurgency in its northwest tribal region, but attacks in Punjab and Lahore -- the country's political heartland that will be vital in approaching general elections -- have been rare in recent months.
But on Monday, gunmen shot dead seven security personnel at an army camp less than 150 kilometres (100 miles) southeast of Islamabad, again arriving by motorbike, opening fire and then fleeing.
One senior security official told AFP it was "highly likely" that the attackers belonged to a banned organisation in league with the Taliban.
Tensions have been high among right-wing and extremist organisations since Pakistan last week decided to reopen its Afghan border to NATO supply convoys, ending a seven-month blockade following negotiations with US officials.
The Defence Council of Pakistan, a coalition of right-wing and hardline Islamist groups, bitterly opposes the country's alliance with Washington and the resumption of supplies for NATO troops fighting the Taliban in Afghanistan.
Hafiz Mohammad Saeed, the founder of the banned Islamist group Lashkar-e-Taiba blamed for the 2008 attacks in Mumbai, who has a $10 million US government bounty on him, has urged Pakistanis to protest.
Just hours before Monday's attack, thousands from the Defence Council of Pakistan passed through the area on a "long march" from Lahore to Islamabad to demonstrate against the reopening of NATO supply routes.
Pakistan closed the routes in protest at US air strikes last November that killed 24 Pakistani soldiers.
The air raid plunged ties between the Islamabad and the United States, already shaky after the US raid that found and killed Osama bin Laden in a Pakistani garrison town in May 2011, to a new low.
After months of negotiations, a rapprochement was achieved when US Secretary of State apologised for the deaths in the air raid.