At least 35 people have been killed in an explosion in Pakistan during a rally organized by an Islamist party. At least 200 people were also injured in the explosion in northwest Bajaur district, where Jamiat Ulema-e-Islam-Fazl (JUI-F) was holding a meeting. Police told the BBC that they have found evidence suggesting the explosion was a possible suicide attack. A rescue operation has been completed and all injured have been taken to hospital, officers said. Security forces have cordoned off the area and government officials have warned the death toll may rise further as many of the injured are critical. The motivation behind the attack is not yet clear. Hundreds of people were attending the JUI-F workers" convention on Sunday in the town of Khar, in the Pakistani tribal district of Bajaur, Khyber Pakhtunkhwa province, near the border with Afghanistan. Images being broadcast on local TV show ambulances ferrying injured people to hospital, with some being flown by military helicopters to nearby Peshawar for treatment. Authorities have declared a health emergency at the district hospital. Some badly injured people have been waiting in the hallways of health clinics struggling to cope with the high number of casualties. A regional leader of the JUI-F, Maulana Ziaullah, was killed in the blast, local officials told the BBC. JUI-F is a major religious political party and forms part of the government coalition in Pakistan"s parliament. The party"s leader, Fazal-ur-Rehman, has demanded that Pakistan"s Prime Minister Shehbaz Sharif launch an inquiry into the explosion. The political gathering was an opportunity for the JUI-F to rally its support, ahead of an election expected to take place later this year. While no-one has yet said they carried out the attack, Pakistan"s Islamic State group (IS) claimed to be behind several attacks this year in Bajaur and has previously claimed targeting JUI-F. In June, the militants said they were behind the assassination of a party official in the village of Inayat Killi. IS issued the claim in the name of its "Khorasan Province" branch (ISKP), which operates in Afghanistan and north-west Pakistan. The group also claims attacks in Pakistan in the name of its "Pakistan Province" branch.