"It was a powerful blast, at least six rooms were demolished," he said, adding that one bus and two police vehicles were damaged.
No one claimed responsibility for the attack, but Taliban fighters have targeted police, security forces and government offices in various parts of Pakistan in recent years in response to military operations in the country's tribal regions.
The attack in Karak, located 200km southwest of Islamabad, the capital, came as police were on the alert for attacks on religious processions to mark the anniversary of the Prophet Muhammad's birth.
In the northwestern district of Dera Ismail Khan, armed men opened fire on an Eid Milad un-Nabi procession, killing one man and wounding several others, officials said.
It also came a week after teams of suicide bombers attacked two police stations in the northwestern district of Mansehra, killing an area police chief and wounding several officers.
The Pakistani Taliban, allies of the Afghan Taliban, have lost much ground in army offensives over the past year.
The were pushed out of the Swat valley, northwest of Islamabad, and in October the army began a big offensive in the fighters' South Waziristan bastion on the Afghan border.
Pakistani action against the fighters on the border is seen as crucial for efforts to bring stability to Afghanistan, where US forces are spearheading one of Nato's biggest offensive against the Afghan Taliban.
At least 57 people were killed in Karachi in an attack on a Muslim congregation marking the holiday in 2006.