The explosion took place on GT Road in the Nishterabad area of Peshawar.
A police mobile was also reported to have caught fire after the grenades were launched, according to eyewitness reports. A police official said that initial reports suggested that grenades were hurled directly at the police van.
The four injured policemen have been shifted to Lady Reading Hospital.
Security officials have cordoned off the area and GT Road has been blocked.
The attack took place just a few hours after the new governor of Khyber Pakhtunkhwa Engineer Shaukatullah was sworn in earlier on Sunday.
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