Shiites have been increasingly attacked by extremist groups that view them as heretics and non-Muslims in this Sunni Muslim-dominated country. Many of the Shiites in Quetta, including those in the neighborhood attacked Saturday, are Hazaras, an ethnic group that migrated to Pakistan from Afghanistan more than a century ago.
Zubair Mahmood, Quetta's police chief, told reporters that the bomb was hidden in a water tank and towed into the market by a tractor. He said the blast destroyed shops in the neighborhood and caused a two-story building to collapse.
"We fear some victims may be found buried there," he said.
Mahmood said police did not know who was behind the bombing. A local television station reported that Lashker-e-Jhangvi, a Sunni extremist group that has targeted Shiites in the past, had called to claim responsibility.
Senior police officer Wazir Khan Nasir said the bomb, set off in a residential suburb, was detonated by remote control.
Another officer, Samiullah Khan, said the bomb was detonated while dozens of women and children were buying produce for their evening meal. Residents rushed the victims to three area hospitals, often in private vehicles because there were not enough ambulances.
A huge plume of white smoke rose over the area after the bomb blast. Television footage of the scene showed the streets littered with rubble from destroyed buildings, mixed with fruits and vegetables and shattered street carts.
Near one of the hospitals, a man stood weeping as people were being taken in on stretchers.
"Look at our misery! We are helpless," he said.
Members of the minority Shiite sect took to the city's streets in angry protest, blocking roads with burning tires and throwing stones at passing vehicles.
Many also started firing into the air in an attempt to keep people away from the area in case there was a secondary explosion. Sometimes insurgents stagger the explosions as a way to target people who rush to the scene to help and thus increase the death toll.
Police cordoned off the area. Shiites in the area were quick to blame Lashker-e-Jhangvi.
"This evil force is operating with the patronage of certain elements in the province," said Qayum Changezi, the chairman of a local Hazara organization.
Saturday's attack was the worst since a series of bombings Jan. 10 killed 86 people in Quetta, almost all Hazara Shiites. Residents were so furious that they refused to bury their dead for days, instead camping out on the streets with the bodies in coffins in protest and demanding the government address the problem.
After days of protests, Prime Minister Raja Pervaiz Ashraf flew to the city to meet with protesters and sacked the chief minister and his cabinet. But Saturday's attack showed the still potent power of the extremist groups behind such incidents.
Quetta is the capital of Baluchistan province, where the Shiite minority has been attacked several times in recent months. Baluch nationalist groups are fighting an insurgency there to try to gain a greater share of income from the province's gas and mineral resources. Islamic extremists are also active in the province.
Lashkar-e-Jhangvi took its name after a firebrand Sunni cleric who gave virulently anti-Shiite sermons.