The explosion in Bajaur district, where Jamiat Ulema-e-Islam-Fazl (JUI-F) was having a meeting, also injured more than 100 persons.
There has been no claim of responsibility for the attack, which is being looked into. Uncertainty surrounds a cause.
The death toll could increase further, according - officials, as many people are in critical condition.
According to the officials, the rescue effort is over, and all injured people have been brought to a hospital.
The area has been blocked off by security personnel while the inquiry is being conducted. The explosion was a suicide bombing, according to Khyber Pakhtunkhwa's police chief. Approximately 10kg of explosive material were utilised in the attack, according to bomb disposal officials.
On Sunday, hundreds of people gathered in the town of Khar, in the tribal region of Bajaur, Khyber Pakhtunkhwa province, close to the Afghan border, for the JUI-F workers' convention.
Hundreds of people could be seen gathered beneath a canopy when the blast occurred not far from the stage, according to pictures.
According to one eyewitness, the tent collapsed, trapping those who were frantically attempting to flee.
Ambulances are seen transporting injured individuals to hospitals in images seen on local television, and police stated that those who sustained critical injuries had been transported by military helicopters to the adjacent city of Peshawar for immediate medical attention.
The district hospital is now under a state of emergency, according to the authorities.
While medical facilities struggled to deal with the large number of casualties, several seriously injured persons were waiting in the hallways.
Local authorities told the BBC that Maulana Ziaullah, a regional JUI-F leader, died in the explosion.
A significant religious political party, JUI-F is a member of the coalition that governs Pakistan's parliament.
Shehbaz Sharif, the prime minister of Pakistan, blasted the perpetrators as terrorists who "targeted those who speak for Islam, the Quran, and Pakistan" and warned that they would suffer "real punishment."
The prime minister declared in a statement that "we will eradicate the terrorists from the face of existence."
The political event gave the JUI-F a chance to gather support before an election that is anticipated for later this year.
Although no one has claimed responsibility for the attack as of yet, the local Pakistani affiliate of the Islamic State (IS) organisation has previously claimed responsibility for multiple strikes in Bajaur this year that were allegedly directed at JUI-F.
The militants claimed responsibility for the murder of a party official in the village of Inayat Killi in June.
The assertion was made by IS on behalf of its "Khorasan Province" (ISKP) branch, which conducts business in Afghanistan and northwest Pakistan. In the guise of its "Pakistan Province" branch, the group also claims responsibility for assaults in Pakistan.
The Pakistani Taliban (TTP) denounced the assault and said they had no part in it.
Since the Afghan Taliban seized power in neighbouring Afghanistan in 2021, attacks by the TTP and other militant groups have increased once more in north-western Pakistan. After breaking a ceasefire in November of last year, the TTP started attacking again.