The bombing targeted Abdul Ghafoor Haideri, deputy leader of the country's Senate, when he came out of a religious seminary after addressing a gathering in Mastung area of the province shortly after the prayer.
Haideri was wounded in the blast.
Senior police officer Abdul Razzaq Cheema said that 25 people were killed in the attack and another 35 were wounded.
Several victims were reported to be in critical condition.
No one claimed responsibility for the attack. An investigation is underway.
Haideri later told Dunya TV in a brief chat that he received minor injuries.
TV footage showed Haideri s vehicle was badly damaged. He was sitting in the car at the time of blast. His driver was killed in the attack.
Haideri belongs to Jamiat Ulema-e-Islam Fazal (JUI-F) of Maulana Fazlur Rehman.
(With inputs from agencies)