At least 20 people have been killed and 100 injured after a car bomb tore through a gas station in the Pakistani city of Faisalabad.
Police said the device caused several gas cylinders at the station to explode, destroying cars and buildings.
Officials believe the nearby offices of Pakistan's ISI intelligence agency may have been the target.
Until now, the city of Faisalabad - a key hub for Pakistan's textile industry - has rarely been targeted.
The BBC's M Ilyas Khan, in Islamabad, says that although ISI offices and Pakistani military bases have been frequently targeted by militants elsewhere in Pakistan, this is thought to be the first attack on a security establishment in Faisalabad.
In recent years hundreds of people have been killed in militant attacks across Pakistan, though they more commonly occur in the north-west of the country near the restive tribal areas along the Afghan border.
Prime Minister Yousuf Raza Gilani has expressed his sorrow over the deaths, according to state-run news agency APP.
'Like an earthquake'
"It was not a suicide attack. It was a planted bomb blast. The bomb exploded near the gas cylinders that triggered a bigger blast," commissioner Tahir Husain told Pakistani TV station, Waqt.
Many vehicles in Pakistan run on compressed natural gas.
A security guard at the nearby office of Pakistan International Airlines said: "We heard the explosion from the gas station, the earth moved as in an earthquake, and there was thick smoke all around."
Television footage showed collapsed buildings and burnt-out cars.
"There are some people trapped under the building rubble. We have deployed our cranes and machinery to rescue them very soon," Mr Husain told Pakistan's private Geo television.
Regional police chief Aftab Cheema said: "We fear the death toll may rise because the condition of some of the injured are very critical."
