Assistant political agent of central Kurram region, Mohammad Fazal said at least 15 people had died in the incident in the rally called by the right-wing Jamiat Ulema-i-Islam-Fazl (JUI-F) in Kurram tribal region.
According to reports, nearly 70 people were injured.
The explosion occurred when a planted device detonated inside the madarsa where the rally was taking place, political agent Riaz Masood said.
The rally of JUI-F's Munir Khan Orakzai, a former lawmaker from the Federally Administered Tribal Areas (FATA) who is contesting the upcoming elections, was taking place when the explosion occurred, The Dawn reported.
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Sources said that another JUI-F candidate Ainuddin Shakir, was also injured in the blast, the report said.
Emergency and rescue forces reached the spot and shifted the victims to nearby hospitals. Security forces cordoned off the area as a probe into the incident went underway.
It was the first deadly attack on a political party in Pakistan's tribal belt since campaigning began for Pakistan's polls, which mark a historic democratic transition in a country ruled by the military for half its life.
JUI-F Chief Maulana Fazlur Rehman also condemned the killing of innocent people.
Orakzai had recently joined the JUI-F.
No organisation has immediately claimed responsibility for the attack.
A JUI-F spokesman told the media that around 3,000 people, including tribal elders were at the meeting at the time of the blast.
Violence has risen sharply before the vote with more than 60 people killed in direct attacks on politicians and political parties since April 11.