The explosion took place near a police truck parked on Zarghoon Road, in the city's high-security Red Zone, 300 metres from the provincial assembly building, Geo News reported.
Later, the provincial assembly session was adjourned after Sanaullah Zehri made a decision to step down on the advice of Prime Minister Shahid Khaqan Abbasi.
The Pakistani Taliban claimed responsibility for the attack, saying a suicide bomber targeted a police contingent returning from duty at the assembly building.
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On Sunday she tweeted: "In 2015 we introduced the 5p charge on plastic carrier bags, we now see 9bn fewer bags being used". The ban has been welcomed by campaigners and cosmetic companies which already use natural alternatives to microbeads .
The security vehicle was targeted some 250 yards from the Balochistan Assembly at Zarghoon Road where around 6,000 personnel of the law-enforcing agencies were deployed in the adjacent areas for the key assembly session.
Doctors and rescue workers said the victims were mostly police personnel and several of those injured were in "critical condition".
The injured have been shifted to Civil Hospital and Balochistan Medical Complex (BMC), while emergency has been declared in these medical facilities.
Police said the explosives weighed between 15 to 20 kilogrammes.
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The blast also damaged a local bus, three vehicles, a rickshaw and two motorcycles.
Security officials said a bomb disposal team was present at the location conducting necessary search and sweep exercise.
Senior police officer Abdul Razzaq Cheema confirmed the deaths of the four police personnel who belonged to the Balochistan Constabulary force.
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