Assistant Political Agent of central Kurram region, Mohammad Fazal said at least 15 people had died in the incident. Reports said at least 70 people were injured.
Political agent Riaz Masood said the explosion occurred when a planted device detonated inside the madressa where the rally was taking place.
A 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.
Sources also 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.