Muhammad Idrees Khan, the deputy inspector general of police in Lower Dir, told the AFP news agency that the mosque hit in the attack was next to a local police headquarters.
"The bomber was riding a car and he rammed his car into the outer gate of the mosque," he said.
Wakeel Mohammad Khan, the head of the main hospital in Lower Dir, said that 29 people had been brought in, some of them in a critical condition.
"The local residents told me that some more dead bodies are laying in the houses - we have declared an emergency in the hospital," he said.
The army offensive launched in North West Frontier Province in April killed at least 2,150 fighters across the three districts, according to the military.
However, despite the military declaring victory after three months, there continue to be sporadic attacks across Swat, Lower and Buner.
Meanwhile, three air raids by a suspected US drones in North Waziristan over 24 hours left at least 23 people dead.
Officials said that the attacks hit at least two suspected compounds and a car.
The US does not confirm attacks in Pakistan by unmanned aircraft, but the US military and the CIA are the only operators of such vehicles in the region.