Police official Safar Khan told AFP news agency it was not clear whether it was a bomb or a suicide attack.
Many other people were reported to have been injured in the blast.
Senator Haideri told Reuters news agency he sustained minor injuries.
"My convoy was going to Mastung and there was an explosion. There are many casualties as there were many people in the convoy," he said.
The most severely injured were air-lifted to hospital in the provincial capital, Quetta.
Senator Haideri represents the Jamiat Ulema-e-Islam Fazl (JUI-F) party and most of those injured or killed were party workers, Pakistani media reports said.
It was not clear who carried out the attack. Separatist militants have been fighting in Balochistan for decades. Islamist extremist groups also operate in the province.
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