Another 150 worshippers were wounded in Quetta on Tuesday, according to hospital sources.
Guns and bombs were used in the attack, which came as Shia held a procession to commemorate Ashura, marking the death of Husayn, an important figure in Shia history and grandson of the Prophet Muhammad.
In a separate incident, one Shia was killed and more than 30 hurt in a clash with Sunni Muslims in a village in the province of Punjab, police said.
The attack and the clash in Pakistan came as at least 140 people were killed in a series of bomb blasts in Iraq's holy Shia city of Karbala and in the capital Baghdad.
Information Minister Shaik Rashid called the incident a "sectarian attack".
"It is a sectarian attack and some arrests have been made in this connection," he said.
The violence triggered a rampage by mobs of Shia Muslims who torched shops, burnt tyres and blocked roads.
It was unclear whether Tuesday's victims were shot by attackers or police and paramilitaries firing indiscriminately after an explosion and gunshots from the top of a building triggered stampedes.
"We don't know as yet how they were shot... because there was a stampede and there's no confirmation who fired and what's going on and there's still firing in the city," Quetta mayor Muhammad Rahim Kakar said.
"...Police joined in the firing without realising where gunfire was coming from..."
Unnamed intelligence official
Witnesses said a gunshot was fired from the roof of a building into a procession of thousands of Shia Muslims observing Ashura, the holiest day of their year, in Quetta's busy Meezan Chowk area at around 1:40 pm (0840 GMT).
"It was followed by an explosion in the procession that triggered panic and people starting fleeing," Kakar said.
Paramilitaries and police guarding the procession fired at the building where the gunshot came from and fired into the air, Kakar said.
Some Shia Muslims in the parade also pulled out guns and fired in the direction of the first gunshot, a Quetta-based intelligence official said.
The result was a "three-way firing" between security officers, unknown attackers and Shia devotees, he said, asking not to be named.
"From within the procession people retaliated with guns. Police joined in the firing without realising where gunfire was coming from, people did not have a clue who was firing at who. That led to complete chaos and a stampede," he said.