The explosion, a suspected suicide blast, rocked Quetta's Zarghoon road located in the city's high-security Red Zone, around 300 metres from the provincial Assembly building, Geo news reported.
A special session of the Assembly had been called to take up a no-confidence vote against Balochistan Chief Minister Sanaullah Zehri but was postponed after he resigned from his post, amid speculation of 'horse trading' as the government and the Opposition had claimed to have enjoyed the support of majority of leaders.
Hospital sources confirmed the death toll, saying at least six persons, including four policemen, were killed and at least 17 others injured in the explosion, the report added.
Police officials did not disclose the exact nature of the blast. However, according to sources, initial investigation suggested a suicide attacker on a motorcycle was attempting to target the Balochistan Assembly building but detonated his explosives near the high-security Red Zone area.