In the first incident, gunmen forced the pilgrims going to Iran out of a bus, lined them up and sprayed bullets killing 26 of them on the spot and leaving six more seriously injured.
This is was one of the most brutal attacks on the minority community in the recent past.
The bus was going from provincial capital Quetta to the border town of Taftan.
The injured were taken to hospitals in Quetta, located about 40 km from Mastung.
Shortly after the attack on the bus, militants ambushed a vehicle carrying Shias at Akhtarabad on the outskirts of Quetta, killing two men. Several others were injured.
The Shias were on their way to Mastung to collect the bodies of their kin.
The militants who carried out both attacks escaped.
The banned Lashkar-e-Jhangvi claimed responsibility for the attack on the bus in calls made to newspapers in Quetta.
Mastung is considered a hub of the LeJ, a notorious sectarian group that has carried out numerous attacks on Shias across Pakistan.
Khushal Khan, the driver of the bus, told reporters that the attackers blocked the road with a pick-up truck and ordered passengers to get out of the vehicle.
"The attackers were carrying Kalahnikovs and rocket launchers. After they opened fire, some of us escaped by running away," Khan said.
Though security cover is usually provided to vehicles carrying Shia pilgrims, Balochistan Home Secretary Naseebullah Bazai said the pilgrims had not informed the Home Department about their journey or sought security.
Khan, the bus driver, said about 45 people were in the vehicle.
He said security forces arrived at the site an hour after the attack.
The Shias were travelling to Taftan, a border town from where they intended to cross over to Iran for a pilgrimage.
Members of the minority Shia community have been repeatedly attacked in Balochistan, including in the provincial capital Quetta.
President Asif Ali Zardari condemned the attack on the bus, describing it as an inhuman and brutal act of terrorism.
He said such cowardly acts cannot dent the government's resolve to fight terrorism to the finish.
