Baluchistan Home Minister Agha Omer Bangulzai told Reuters that the death toll in the attack had risen to 128 people, with over 150 wounded.
"It apparently was a suicide attack", he said, though that could not be immediately confirmed.
Police had earlier said that the attack targeted Raisani's convoy but later changed their statement as video footage of a large tent showing damage from the blast was circulated.
Nawabzada Siraj Raisani, who was campaigning for an assembly seat in the southern province of Balochistan, was killed in the bomb blast along with dozens of others, Balochistan Home Secretary Haider Ali Shikoh told CNN.
"Raisani succumbed to wounds while he was being shifted to Quetta", District Police Officer Mohammad Ayub Achakzai said. Caretaker Chief Minister Balochistan Allauddin Marri also attended funeral prayer.
Pakistan Prime Minister's Office, in a statement, today said in view of tragic incidents of terrorist attacks at Peshawar on Tuesday and in Mastung and Bannu yesterday, the government has announced to observe one day official mourning throughout the country tomorrow.
The blast in Mastung was the third election bomb attack this week and followed the killing of four hours earlier at a rally in Bannu.
The Islamic State claimed the bombing, according to group's Amaq propaganda agency.
The military has already warned of a security threat in the run-up to the vote, and said it will deploy more than 370,000 soldiers on polling day.
He claims he is being targeted by the military, which remains the country's most powerful institution, and which has faced blistering allegations that it is pressuring the media and politicians in a bid to manipulate the polls against the PML-N.
Police were not sure about the nature of the explosion however some local channels quoted eyewitnesses terming the incident as an "act of a suicide bomber".
The military denied the accusations saying their assistance in carrying out the elections was requested by Pakistan's Election Commission.
Balochistan's Civil Defence Director Aslam Tareen said 16 to 20 kg of explosives and ball bearings were used in the attack. "What credibility will these elections have when the government is taking such a drastic action against our people?"
Senior politician Akram Khan Durrani, a candidate of the five-party religious alliance Muttehida Majlis Amal (MMA), was an apparent target of the attack but escaped unscathed, local broadcaster ARY News reported.
The outlawed Pakistani Taliban claimed responsibility for the Peshawar attack, condemning Bilour and his predominantly secular Awami National Partyas "enemies of Islam".
A week of bombings on political rallies has shattered the relative peace of Pakistan's general election campaign, culminating in a devastating suicide attack that killed at least 130 people at a rally in the southwestern Baluchistan province.