The scene of an apparent suicide car bomb attack on a police building in Lahore on May 27.
The bomb brought down a government ambulance service building and damaged a nearby office of the military's main intelligence agency.
"A car came and broke through a barrier and exploded," senior city official Sajjad Ahmed Bhutta said.
Just before the blast two men got out of a car and opened fire at police guards at the gate, witnesses said.
There was no claim of responsibility, but the blast comes as the army is battling militants in the Swat Valley region in the northwest of the country - its most concerted action to push back a growing Taliban insurgency.
"I believe that anti-Pakistan elements, who want to destabilise our country and see defeat in Swat, have now turned to our cities," Interior Ministry chief Rehman Malik said.
The attack came as General David Petraeus, head of the US Central Command, was in Islamabad for meetings with government and military leaders. An office of the military's main intelligence agency is also near the site of the blast.
The US needs Pakistani action against militants in its northwest to defeat al-Qaeda and disrupt support for the Taliban in Afghanistan.
Militant violence has surged in Pakistan since mid-2007, with numerous attacks on the security forces, as well as government and Western targets.
Officials have warned that militants might launch bomb attacks in retaliation for the offensive in Swat where the military says about 15,000 members of the security forces face 4,000-5,000 militants.
Lahore is the capital of Punjab province, the country's most populous and prosperous province. The country's second biggest city is also traditionally home to top bureaucrats and top military officials.
The city has seen several bomb attacks over the past couple of years but had felt much safer than other parts of the country until March when militants launched two attacks.
Attackers firing rifles and throwing grenades stormed a police training academy on the outskirts of the city on March 30, killing eight recruits, wounding scores and holding off the security forces for hours.
That attack, claimed by Pakistani Taliban leader Baitullah Mehsud, came less than a month after a dozen gunmen attacked Sri Lanka's cricket team in the city, killing six police guards and a bus driver.
-Yahoo news
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