The death toll following a blast at an election rally in southwestern Pakistan has risen to 128 people, officials have said.

Balochistan province's home minister said the death toll in the town of Mastung "has risen to 128".

A senior provincial government official also confirmed the figure, adding that 150 others were injured in the attack.

The bombing was the most deadly attack in Pakistan in over a year and is the third incident of election-related violence this week.

It comes amid growing tension over ousted prime minister Nawaz Sharif's return ahead of the 25 July vote.

Mr Sharif, who faces ten years in prison for corruption, was arrested on his return to the country today.

It came as Pakistan's caretaker government launched a crackdown on political gatherings with Mr Sharif, who was ousted by the Supreme Court last year and convicted in absentia a week ago.

A senior police official had earlier said that more than 1,000 people were in attendance at the rally.

Islamic State claimed responsibility for the attack, the group's AMAQ news agency said. The group provided no further detail or evidence for its claim.

Islamist militants linked to the Taliban, al-Qaeda and Islamic State have been operating in the province, which borders Iran as well as Afghanistan.

It also has an indigenous ethnic Baluch insurgency fighting the central government.

Among those killed was Baluchistan provincial assembly candidate Siraj Raisani, whose brother Nawab Aslam Raisani had served as the provincial chief minister from 2008 to 2013.

Mr Raisani is the second electoral candidate to be killed in pre-election violence this week.

Police had earlier said that the attack targeted Mr Raisani's convoy but later changed their statement as video footage of a large tent showing damage from the blast was circulated.

Earlier in the day, a bomb blast killed four people in the northern town of Bannu when it struck the campaign convoy of Akram Khan Durrani, an ally of Mr Sharif's party from the religious Muttahida Majlis-e-Amal party (MMA).

A suicide bomber blew himself up at a rally by an anti-Taliban political party in the northern city of Peshawar on Tuesday, killing 20 people.

The Pakistani Taliban claimed responsibility for the attack.