A suicide bomber targeted an open-air market in the Pakistani city of Quetta, killing at least 20 people and wounding dozens of others, police and hospital officials say.

Shortly after the bombing struck near a Shi residential area on Friday, dozens of angry Shi'ite youths rallied in Quetta, demanding more security from the authorities and the arrest of those behind the attacks.

They also denounced the violence by Sunni extremists who have killed hundreds in similar attacks over the past years in Baluchistan province, where Quetta is the capital.

"It seems people from the (Shi'ite) Hazara community were the target," said senior police chief Abdur Razzaq Cheema, adding that some of the victims were in critical condition.

Mir Ziaullah Langau, the provincial home minister, said the suicide bomber had walked up to the marketplace and killed both Shi'ites and Sunnis.

No group immediately claimed responsibility for the attack, which took place in the Hazarganji neighborhood, but Sunni militant groups have claimed similar bombings in the past against Shias, whom they view as apostates deserving of death.

Cheema, the police chief said at least eight Shias were among the dead, as well as a paramilitary soldier and seven other people.

Four paramilitary troops, who were guarding the open-air fruit and vegetable market, were among the wounded.

Qadir Nayil, a leader of the Hazara community, asked the government to provide better protection for the Shia.

"We demand more security from the government and all those involved in today's act of terrorism should be found and punished," he said.

Pakistani Prime Minister Imran Khan and President Arif Alvi issued statements condemning the attack and adding that it would not weaken "the resolve of the nation in the fight against terrorism".

Jam Kamal Khan, the chief minister of Baluchistan province, also condemned the attack, saying "the enemy of humanity" is to blame.