Lahore, Pakistan, July 13, 2018 (AFP)

Pakistan authorities locked down parts of Lahore Friday for the return from London of former premier Nawaz Sharif, who faces possible arrest and a 10-year prison sentence ahead of already tense elections his party insists are being rigged.

Sharif's arrival was set for a day marked by a spike in violence, with two attacks on two campaign rallies in separate parts of the country killing nearly 40 people and raising fears for security days ahead of the July 25 polls.

Sharif was sentenced in absentia last week to 10 years in prison by a corruption court over the purchase of high-end properties in London, dealing a serious blow to his Pakistan Muslim League-Nawaz (PML-N) party's bid weeks ahead of the vote.

He has claimed he is being targeted by the military.

"I know that... I will be directly taken to jail," he said in a video released by his party Friday.

"I want to tell Pakistanis I have been doing this for you... Walk with me, join hands with me and change the destination of the country."

Pakistani reporters travelling with Sharif said later Friday that he had boarded a flight in Abu Dhabi for his last leg to Lahore.

An estimated 8,000 supporters have gathered there for a rally led by his brother Shahbaz Sharif, who now heads the PML-N. Shahbaz has said they will remain peaceful.

Roughly the same number of police are expected to be deployed, police officials told AFP earlier, while some containers were set up blocking roads, and travel to and from the airport was reportedly restricted.

On Thursday Shahbaz said that hundreds of the party's workers and supporters had been arrested in what he described as "naked" pre-poll rigging.

Despite the crackdown, PML-N backers continued to flock to Lahore earlier Friday, vowing to confront authorities if challenged.

"We are going to airport and if anybody stops us then we are ready to go to jail," Khurram Ehsan, 36, told AFP.

"We can go beyond our limits for our leader."

Security fears

Fears of violence were also spiking ahead of the polls after the two bomb blasts Friday underscored the fragility of Pakistan's dramatic improvement in security.

A bomb killed at least 25 people, including a local politician, and injured more than 30 at a rally in restive southwestern Balochistan province. Authorities said it may have been a suicide attack, and no group immediately claimed responsibility.

Earlier, four people were killed by a bomb targeting another politician's convoy in the country's northwest. The politician survived, but 39 others were wounded, officials said. There has been no claim of responsibility.

On Tuesday a bomb claimed by the Pakistani Taliban targeted a political rally in the city of Peshawar, killing 22 people.

Nawaz left London late Thursday, where his wife is being treated for cancer.

Analyst Zahid Hussain said the ex-leader was "fighting back for his political life".

"Certainly it would have been worse for the party's prospects had he not come," Hussain added.

"In Pakistan, leaders have to go to jail, that's important for their profile," said Pakistani journalist Rahimullah Yusufzai.

He added that dramatic images of Nawaz leaving behind an ailing wife in London to return to Pakistan and face arrest are all but certain to boost his image in the eyes of his supporters.

Since Nawaz returned to London last month, the PML-N has fallen into disarray and failed to mount an organised campaign to rally the base ahead of the polls.

The election will pit the PML-N against its main rival, the Pakistan Tehreek-e-Insaf, which is led by cricket star-turned-politician Imran Khan.

Recent polling has showed PTI's popularity steadily rising and closing the gap with the PML-N.

Nawaz was the 15th prime minister in Pakistan's 70-year history -- roughly half of it under military rule -- to be removed before completing a full term.

Sharif has faced -- and fought off -- similar challenges in the past.

In 1993 he was sacked from his first term as premier for corruption, while in 1999 he was sentenced to life in prison after his second term in office ended with a military coup.

Following the coup he was allowed to go into exile in Saudi Arabia, returning in 2007 before becoming prime minister for a third time in 2013.

The military remains the most powerful institution in the country, and has faced blistering allegations that it is pressuring the media and politicians in a bid to manipulate the polls against the PML-N.

It denies the accusations.