Pakistan's government has blamed Taliban fighters for a bomb attack in Lahore which killed 23 people and left hundreds more injured.
A group of men shot at police officers before detonating a powerful car bomb, damaging buildings belonging to the police and intelligence agency the ISI.
Rescuers are searching the rubble and warn that the death toll could rise.
Officials said the Taliban carried out the attack in revenge for a military offensive against them in Swat valley.
Interior Minister Rehman Malik told reporters: "Enemies of Pakistan who want to destabilise the country are coming here after their defeat in Swat.
"There is a war, and this is a war for our survival."
'Attack on the state'
At least one ISI agent, 12 police officers and one child were reported killed in the attack, at about 1030 local time (0530 GMT).
Local officials have speculated that the military intelligence agency could have been the target.
I ran out of the building and saw a surreal huge ring of white smoke rise into air
'Everything shattered'
The ISI's offices were damaged by the bombing, and a police emergency-response building was flattened.
The BBC's Owen Bennett-Jones, in Lahore, says it is not clear which organisation the perpetrators were attacking - but it is clear that they were attacking the Pakistani state.
A least two arrests were made, but police officials later told the BBC that those detained appear to have been innocent bystanders.
Meanwhile, the military says it expects the main town in the Swat valley, Mingora, to be cleared of Taliban insurgents within two or three days.
The military said two other areas away from Swat which have also seen heavy fighting - Mohmand and Sultanwas - were now safe enough for residents to return home.
Sustained violence
Lahore, in Punjab province near the Indian border, is known as Pakistan's cultural capital and is far from the Swat valley.
But in March militants laid siege to a police compound in the city, killing eight people, and weeks earlier the Sri Lanka cricket team was attacked there.
The BBC's Shoaib Hasan, in Pakistan, says Lahore is facing a sustained campaign of violence unlike any it has seen before.
He says security officials believe the city is under attack because it is seen as a stable home for Pakistan's Punjab-dominated army.
The army is claiming sweeping victories against Taliban insurgents in the Swat valley, near the Afghan border - saying more than 1,000 militants have been killed in the past month.
Militants had threatened revenge attacks in Pakistan's cities after the military stepped up its operations in the Swat valley.
Global condemnation
After the latest attack, television footage showed rescue workers sifting through the debris, pulling half-conscious police officers from the rubble.
Bulldozers and other heavy lifting equipment have been brought in as many people are feared trapped under the debris.
Officials told reporters a car pulled up near the police headquarters and a group of gunmen got out and opened fire.
When police returned fire, the gunmen's car exploded.
BBC News website readers in the city described hearing a huge explosion.
Zubair Bukhari, who was in his office about 500m away from the blast, said it rocked the entire building.
"Glass windows shattered to pieces and the ceiling came down on the floor," he said.
Another reader, Matthias Gattermeier, said: "I ran out of the building and saw a surreal huge ring of white smoke rise into air."
Politicians from around the world have condemned the attack and offered condolences to Pakistan.
US ambassador Anne Patterson said the attacks "show the lengths extremist elements are willing to go to as they attempt to force their agenda on to a people who only wish to go about their daily lives in peace".
