Two suicide bombers blew themselves up outside Pakistan's main military arms factory, killing 57 people and piling new pressure on the shaky coalition government to tackle militancy.
The attackers struck almost simultaneously as a crowd of workers was streaming out of the huge factory complex in the northern town of Wah, near Islamabad. It is one of the country's most sensitive installations.
The Taliban claimed responsibility for the attack, the second to rock Pakistan since Pervez Musharraf resigned as president on Monday, and warned of further blasts if army operations near the Afghan border are not stopped.
"It's a massive attack," local police chief Nasir Durrani told AFP. "Two men apparently blew themselves up outside the factory during a shift change. The bombers were on foot and they exploded themselves less than a minute apart."
He said that 57 people were confirmed dead and around 70 wounded.
The charred body of a bearded man, believed to be one of the bombers, lay on the road outside one of the gates. A severed leg, abandoned shoes and several mangled bicycles were scattered nearby.
Dozens of troops, police and military rescue workers in orange jackets milled around the scene.
Prime Minister Yousuf Raza Gilani strongly condemned the attack and "directed the authorities to make efforts to expose the hidden hands behind the incident", the state-run Associated Press of Pakistan news agency said.
A spokesman for Tehreek-e-Taliban Pakistan, the umbrella group for the country's Taliban militants, said they were responsible.
"Our bombers carried out today's attack. It is in reaction to military operations in Swat and Bajaur," spokesman Maulvi Omar told AFP by telephone, referring to two north-western regions where troops are fighting militants.
"Similar attacks will be carried out in other cities of Pakistan including Lahore, Islamabad and Rawalpindi," he said.
The Pakistani Ordnance Factories at Wah is a cluster of about 20 industrial units producing artillery, tank and anti-aircraft ammunition for the Pakistani armed forces. It employs around 25,000 to 30,000 workers.
It adjoins the historic city of Taxila where thousands of tourists come every year to visit ancient Buddhist ruins.
Factory worker Riaz Hussain said most of the victims were labourers who were joining the afternoon shift.
"I was working in the factory when I heard one blast and then another. They were huge," he told AFP. "Security people then immediately surrounded the place and we were not allowed to go outside."
A rescue service official said a fleet of around 25 ambulances was needed to ferry the wounded to hospital.
"The blast took place as staff were leaving after finishing their day's duty and it was very crowded," Zaheer Shah, of Edhi Rescue, Pakistan's largest private charity, told AFP.
The blasts came two days after a suicide bomber attacked a hospital in the northwestern town of Dera Ismail Khan on Tuesday, killing 30 people.
Pakistani forces have been fighting fierce battles for nearly two weeks with Taliban militants in Bajaur, a tribal region bordering Afghanistan. The government says more than 500 militants and 30 soldiers have been killed.
Gilani's government is under massive international pressure to crack down on militants using safe havens in the rugged tribal belt to launch attacks on US and NATO forces in Afghanistan.
But bombings like Thursday's attack have added to public anger and accusations that Pakistan is suffering for its role in what many regard as "America's war".
Around 1,000 people have died in a wave of militant suicide bombings since the siege and storming of the radical Red Mosque in Islamabad in July 2007, in which at least 100 people died.