The bomb was planted in a car on Saryab Road in Quetta and detonated using a remote control, The Express Tribune reported.
"Unknown persons detonated the device by remote control when a patrolling vehicle of [ New Saryab [Police Station] passed by," said Superintendent of Police Saryab Sikandar Tareen
A donkey-cart owner and a rickshaw driver were among those killed and a police officer was among those injured, officials said.
No group has so far claimed responsibility for the attack.
Over the past month, the police have been under frequent attack in Quetta, with seven police officials being killed and several others injured, officials said.
"Police are receiving threats by an armed group that is involved in target killings in Quetta," a senior official said.