'Act of terrorism'
Hayyat would not say who could be behind the attack but said it was "an act of terrorism" and officers were still investigating whether the attack was carried out by a suicide bomber.
Attacks on security forces by fighters linked to al-Qaeda and the Taliban are common in Pakistan in the last two years, but incidents in Mianwali have been rare.
Mianwali is situated on the eastern bank of the Indus river dividing the central province of Punjab from the North West Frontier Province (NWFP) and the tribal lands, where sectarian tensions have in recent years has become rife by the growing strength of Sunni Muslim Pakistani Taliban.
Southern Punjab is home to a number of armed sectarian groups with links to al-Qaeda and the Taleban.
Security forces launched an attack on pro-Taliban groups on Friday near the Khyber tribal region, killing at least 52 people.
Anti-government groups in Khyber have been attacking lorry convoys carrying supplies to Western forces in Afghanistan for the past few months, forcing the US to step up its search for alternative routes.
A bomb killed at least 27 people on Thursday outside a Shia Muslim mosque in the town of Dera Ghazi Khan.
Authorities suspect Sunni Muslim groups, with links to al-Qaeda, were behind sectarian attack aimed at destabilising the state.