The attack on the Picture House cinema in Peshawar, the region's main city, took place as some 90 people were watching a late night show of the film 'Ziddi Pakhtun' (Stubborn Pushtun), police said.
"At least four people have been killed and 31 injured. Attackers hurled two grenades and fled the scene," Faisal Mukhtar, a senior police official, told AFP.
A stampede following the blasts was responsible for many of the injuries, Mukhtar added.
Jamil Shah, a spokesman for Peshawar's Lady Reading Hospital said three dead bodies and 31 injured people had so far been taken to his hospital.
No one has yet claimed responsibility for the attack, but police said that cinema houses in the city were already under threat.
Peshawar is a frontline city in Pakistan's battle against Islamist insurgents, who regard films as sinful.
The Taliban closed down cinemas in the scenic Swat valley in the country's northwest which they controlled from 2007-2009.
The decline of cinema houses in Peshawar has also been accelerated by the advent of videos, DVDs and the Internet and today only seven remain.
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