Officials said the blast occurred at a Jamaiat Ulema-i-Islam-Fazl rally organized by the party's candidates from several election districts in Kurram agency, Geo News reported.
A Jamaiat Ulema-i-Islam-Fazl candidate was among the dead, officials said.
About 3,000 people, including tribal elders from border areas, were at the rally, news reports from the area indicated.
The BBC said the explosion was the latest attack on political parties and candidates since campaigning began last month in advance of the general election Saturday.
The BBC said the majority of the attacks have been against secular parties.
Kurram is one of Pakistan's seven tribal regions near the country's border with Afghanistan.