The Philippine military confirmed six people were killed after two people later died in hospital.
Twenty-seven people were wounded, Lieutenant-Colonel Lyndon Paniza, commander of an army battalion in the area, told reporters, adding that troops were checking hospitals in the vicinity to verify if there were any more victims.
No one has claimed the attack, although suspicion might fall on the Moro Islamic Liberation Front (MILF), which has been fighting the government for indpendence in the south of the country.
Attacks on buses have often been linked to the criminal world seeking to extort money from transport companies.
Three people were killed and 25 were wounded when a crude bomb exploded inside a bus in the same town on July 24.
Darantinan said the owners and managers of a local bus firm operating in the area had been receiving threats after they turned down extortion attempts by a criminal gang known as "al Khobar".