|At least 25 people were killed and many wounded in Saturday's blast at a poll rally in Charsadda [EPA]|
Pakistani police said that they believed fighters are based in the tribal areas of the Afghan border were behind the attack.
Hamid Nawaz, the interior minister, linked the blast to a wave of other bombings blamed on al-Qaeda and Taliban fighters that have killed more than 70 people this year.
The explosion struck a crowd in the town of Charsadda as they gathered to hear Afrasiab Khattak, leader of the Awami National party (ANP), speak.
The ANP is a secular ethnic Pashtun group competing in the legislative elections on February 18.
Khattak told Dawn Television that he had survived the blast.
Farman Ali, deputy mayor of Charsadda, said: "The bomber appeared to be standing in a corner and all of a sudden pushed his way to the middle and blew himself up."
Zahid Khan, an ANP spokesman, said the party thought that the attack was intended to delay or disrupt elections.
The ANP has announced a three-day period of mourning.
Last year, Aftab Ahmed Khan Sherpao, a former interior minister, survived two separate suicide bomb attacks in Charsadda that killed scores of people.
Elections scheduled for January were postponed after Benazir Bhutto, a former prime minister, was assassinated on December 27.
The government has blamed an al-Qaeda-linked tribal leader, Baitullah Mehsud, who is based in the South Waziristan region on the Afghan border, for Bhutto's assassination and many of the other recent attacks across the country.
Mehsud, the most wanted man in Pakistan, has denied the charge.
Kamal Hyder, Al Jazeera's correspondent in Islamabad, said: "The big question is who carried out the attack and why, and why so close to what is being considered the country's most crucial election in its history."
Source: Al Jazeera and agencies