City Police Officer Rao Iqbal said initial investigation suggested that some policemen were on patrol in a private car when probably a grenade was hurled at them from a distance.
All the injured have been admitted to the DHQ hospital, where doctors described one of the injured, Mohammad Imran's condition as critical.
Another injured person, Mohammad Arif, said after receiving a call from his brother that he has been stopped by police on the Misryal road, he (Arif) along with his friends reached the spot for verification of certain documents when suddenly the blast occurred.
"It looked as if lightning had struck from the sky," Arif said.
The Dawn identified the injured policemen as Aftab, Yousaf, Malik Liaquat and Saeed