Ousted Pakistani Prime Minister Nawaz Sharif and his daughter Maryam were arrested on Friday after flying back to the country to face lengthy prison sentences, in a high-stakes gamble to galvanize their beleaguered party ahead of a July 25 election.
It was the second election-related violence to occur on Friday.
Uniformed men escorted the Sharifs, who were sentenced in absentia on corruption charges last week, from their airplane after it touched down in the central city of Lahore at around 8:45 p.m. (1645 GMT), a Reuters reporter on board said.
Sharif is expected to appeal his conviction and seek bail.
Barbed wire was strung across some roads leading to the Lahore airport on Friday and barricades were positioned at the roadside ready to close off main boulevards should crowds start to gather.
The injured were taken to District Headquarter (DHQ) Hospital Bannu for treatment, where officials confirmed the death toll had risen to four while 32 injured were undergoing treatment.
Pakistan Peoples Party (PPP) leaders, Asif Ali Zardari and Bilawal Bhutto Zardari, also condemned the attack.
During his term in office, Sharif criticized the military's involvement in civilian affairs and its efforts in fighting extremists.
Their return could shake up an election race marred by accusations Pakistan's powerful military is working behind the scenes to skew the contest in favor of ex-cricket hero Imran Khan, who describes Sharif as a "criminal" who deserves no support. The army will deploy 350,000 security personnel to polling stations throughout the country on election day.
The attacks underscored the fragility of Pakistan's dramatic improvement in security and raised fears ahead of the vote.
The bomb targeted the convoy of the Muttahida Majlis-e-Amal (MMA) candidate Akram Khan Durrani, who survived the attack, police said. Durrani is standing against cricketer-turned-politician Imran Khan, whose Pakistan Tehreek-e-Insaf party (PTI) is one of the frontrunners in the July 25 vote.
This is the second major incident of pre-poll violence in Khyber Pakhtunkhwa this week.
Durrani's convoy attacked in Bannu. Candidate Akram Khan Durrani had just finished his speech when the bomb exploded.
The blast comes days after a bomb claimed by the Tehreek-e-Taliban targeted a rally by the Awami National Party (ANP) in the city of Peshawar on Tuesday.
"There seems to be a conspiracy to sabotage the 25 July elections but the people of Pakistan will not allow any design meant to target these historic elections to succeed", he tweeted. But he said the people of Pakistan will not allow anything to prevent "historic" elections from taking place.