A suicide bomber blew himself up inside a political rally in southwest Pakistan yesterday killing 128 people, in one of the deadliest attacks in the country's history.

The blast -- which was claimed by the Islamic State group -- ripped through the crowd in the town of Mastung near the Balochistan provincial capital Quetta.

It was the latest in a string of attacks that have spurred fears of violence ahead of nationwide polls on July 25, and underscored the fragility of Pakistan's dramatic gains in security.

"The death toll has risen to 128," Balochistan home minister Agha Umar Bungalzai told AFP. A senior provincial government official also confirmed the figure, adding that 150 others were injured.

Emergency workers shuttled victims to nearby vehicles from the bombed-out compound as bystanders sobbed in the darkness due to the lack of electricity in the impoverished area.

Victims in blood-smeared clothes were taken to hospitals in Mastung and nearby Quetta, where they were greeted by tense crowds of mourners, an AFP reporter said. The deceased could be seen covered in shrouds.

"Human remains and red bloody pieces of flesh were littered everywhere in the compound. Injured people were crying in pain and fear," said local journalist Attah Ullah.

According to senior provincial official Saeed Jamali, the bomber detonated in the middle of a compound where a political meeting was taking place. Another senior official, Qaim Lashari, also confirmed it was a suicide blast.

The explosion killed Siraj Raisani, who was running for a provincial seat with the newly formed Balochistan Awami Party (BAP), provincial home minister Agha Umar Bungalzai told AFP.

"Mir Siraj Raisani succumbed to wounds while he was being shifted to Quetta," he added. Raisani was the younger brother of former provincial chief minister Mir Aslam Raisani.

The attack was the most lethal since Taliban militants assaulted a school in the northwestern city of Peshawar in 2014, killing over 150 people, mostly children, and one of the deadliest in Pakistan's long struggle with militancy.

It came hours after four people were killed and 39 injured when a bomb hidden inside a motorcycle detonated near a Pakistani politician's convoy in Bannu on Friday, near the border with Afghanistan.

The politician -- Akram Khan Durrani, a candidate of the Muttahida Majlis-e-Amal (MMA) party -- survived, police said. No group has yet claimed responsibility for that attack.

On Tuesday, a bomb claimed by the Pakistani Taliban targeted a rally by the Awami National Party (ANP) in the city of Peshawar.

Local ANP leader Haroon Bilour was among the 22 killed. Thousands flocked to his funeral the next day.

The Islamic State group has a muted presence in Pakistan but has carried out brutal attacks there in the past, including the blast at a Sufi shrine in February last year which killed nearly 90 people.

Militants have targeted politicians, religious gatherings, security forces and even schools in Pakistan.

But security across the country has dramatically improved since government and military operations cleared large swathes of territory near the Afghan border in recent years.

Analysts warn, however, that Pakistan has yet to tackle the root causes of extremism, and militants retain the ability to carry out attacks.

The military has warned of security threats in the run-up to the tense election on July 25, and said it will deploy more than 370,000 soldiers on polling day.

Following the series of attacks this week, activists called for Pakistani authorities to remain vigilant to protect candidates during the final days of the campaign season.

"The Pakistani authorities have a duty to protect the rights of all Pakistanis during this election period -- their physical security and their ability to express their political views freely, regardless of which party they belong to," said Omar Waraich, deputy South Asia director at Amnesty International.

Last month, a US air strike killed the leader of the Pakistani Taliban, Maulana Fazlullah, in neighbouring Afghanistan in what the Pakistani army called a "positive development" that also sparked fears of reprisals.