The attack occurred in Timergarah, which is the main town in the Lower Dir district.
Police official Sarzamin Khan said, "It was apparently a suicide attack. The bomber wanted to enter the ground where the ANP (Awami National Party) meeting was taking place but was stopped at the gate and blew himself up."
The ANP heads a coalition government in North West Frontier Province.
Pakistani state television said hundreds of people were participating in the rally in the town of Timergarah.
The critically injured are being shifted to hospitals in Peshawar. The security personnel have cordoned off the affected area.