The blast happened near a mosque in the south-western Balochistan province on Friday as people gathered to celebrate the birth of the Prophet Muhammad.
Officials in Balochistan have declared a state of emergency. No group has claimed responsibility for the attack.
Meanwhile, at least five people died in a separate blast at a mosque near Peshawar City in Khyber Pakhtunkhwa province.
Footage from the scene in the Baloch city of Mastung showed injured people being rescued by emergency responders and locals.
"Bodies were lying on top of one another," said Saifullah, a local journalist in his 40s.
"The crowd was waiting for a [celebratory] procession to begin. I was standing nearby when the bomb went off," he said, adding that 10 to 12 of his relatives died.
Haibullah, 22, was injured in the attack. Speaking to the BBC from his hospital bed in Quetta, the provincial capital of Balochistan, he said: "Everything was ready on the stage. The bomb went off two minutes after I arrived.