There were no immediate details of casualties or the target after the explosion.
Kamal Hyder, Al Jazeera's correspondent in Islamabad, said the political rally appeared to have been the target of the Lower Dir attack.
"There have been a series of attacks against the party, which is in government in the province," he said.
"This is an ongoing thing, although this is a clear warning. Recently the Taliban said there would be more attacks there. So some kind of escalation was expected."