The incident occurred as a suicide bomber rammed his explosive-laden vehicle into the pilgrims' bus in Dareen Ghar area of Mastung district, local media reported.
The bus was carrying over 50 pilgrims who were on their way back from Iran's Taftan city to the provincial capital Quetta in Balochistan, said Shafqat Anwar, assistant commissioner of Mastung, adding the bus caught fire following the blast causing heavy casualties.
Shiite councils across the country condemned the blast and announced a three-day mourning while demanding from the government to provide foolproof security to the pilgrims and Shiite community.
Balochistan's home secretary Asad ur Rehman Gilani said that the provincial government has always provided security to the pilgrims' buses to curb attacks on Shiite pilgrims.
On Tuesday security vehicles were escorting two buses carrying pilgrims when one of the buses was hit by a suicide bomber, he said, adding several security personnel were injured in the blast and two security vehicles were destroyed.
Gilani said 80-100 kg of explosives were used in the attack in which three other vehicles were also destroyed. The bus caught fire following the blast as security forces cordoned off the area, Geo News reported.
No group immediately claimed responsibility for the attack.
Prime Minister Nawaz Sharif and President Mamnoon Hussain condemned the blast.
Sharif lamented over the loss of precious lives and directed hospital administration to provide best possible medical facilities to the sufferers of the blast.