Police official also said that the attack had targeted a local police chief, which killed and injured both police and civilians.
Police spokesman Raouf Ahmedi said the explosion on Monday, Aug 3 in Herat was detonated by remote control, AP reported.
Another police official, Esmatuallah Alizai, told the reporters that the bomb had been left in a roadside dustbin adding, "It exploded as the local chief of district police passed by."
He also added that the two police officials, a woman and child were among those killed, and the local police chief had been seriously injured. Taliban is claimed responsibility for the bomb blast.
Terrorists attacks on security forces have increased in the run-up to the presidential election on Aug 20.