The attackers used automatic weapons and grenades against a crowd of many thousands of Shiites, who make up some 15 percent of the overwhelmingly Sunni Muslim nation of 150 million people.
Shiites rampaged through Quetta after the attack, burning more than 100 shops and sparking fears of a wider backlash.
State-run Pakistan Television said a high court judge had been appointed to head an inquiry and schools in Quetta would remain closed until Saturday.
Hundreds of people have died in violence between Sunnis and Shiites in Pakistan in recent years.
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