ISLAMABAD:  A suicide bomber targeting a political rally in southwest Pakistaninfo-icon Friday killed 128 people, an official said, the deadliest in a string of attacks on campaign events that have raised security fears ahead of nationwide polls. 

The blast -- which was claimed by the ISIS -- ripped through the crowd in the town of Mastung near the Balochistaninfo-icon provincial capital Quetta, and was the deadliest in Pakistan in more than a year.

It came hours after another bomb killed at least four people at a campaign rally in Bannu in the country's northwest. A third bomb killed 22 people at another rally in Peshawar on Tuesday.

The attacks underscored the fragility of Pakistan's dramatic gains in security after years of steady improvement and widespread optimism that things had turned a corner.

The dead included Balochistan Awami Party leader Siraj Raisani.

Siraj, the younger brother of former Balochistan Chief Minister Aslam Raisani, was attacked during a corner meeting organised for the promotion of his candidature. Siraj's death was confirmed by his brother Lashkari Raisani.

After the incident, a large contingent of security forces cordoned off the area with the injured shifted to Civil Hospital Quetta, where an emergency was imposed.

In July 2011, Siraj Raisani lost his 14-year-old son in a terrorist attack in Mastung, in which several others, including security personnel, were killed.

Balochistan's Civil Defence Director Aslam Tareen said 16 to 20 kg of explosives and ball bearings were used in the attack.

Civil Hospital spokesperson Waseem Baig said the hospital received 53 bodies and 73 wounded. At least 20 of those injured were in critical condition.

The Mastung bombing was the latest, and deadliest, of a string of attacks targeting politically exposed persons ahead of the July 25 election.

Strongly condemning the attack, interim Chief Minister Balochistan Alauddin Murree directed Deputy Commissioner of Mastung to submit a report of the incident.

"The attack is a conspiracy to sabotage the electoral process in the province," he said in a statement.

Murree ordered the law enforcement agenciesinfo-icon to ensure those behind the attack were apprehended.

Meanwhile, President Mamnoon Hussain and Interim Prime Minister Nasirul Mulk also condemned the attack in separate statements.

Earlier on Friday, a blast targeted former Khyber-Pakhtunkhwa Chief Minister Akram Khan Durrani's convoy in Bannu. While Durrani remained safe, four people were killed and 32 others injured in the attack.

The Election Commission of Pakistan has postponed polling for the constituency from where Siraj was contesting.

Earlier this week, a suicide blast had killed Awami National Party leader Haroon Bilour and 19 others in Peshawar.

The attack was owned up by the Tehreek-e-Taliban Pakistan.

Before that, on July 7, seven people, including a candidate of Muttahida Majlis-e-Amal, were injured when a convoy came under a bomb attack in Bannu.

There was also an attack on PTI candidate's office in North Waziristan's Razmak tehsil that had injured 10.