The inter-province bus was on its way to Indonesia's capital, Jakarta, from Medan in North Sumatra province when its brakes apparently malfunctioned near a bus terminal in West Sumatra's Padang city, said Reza Chairul Akbar Sidiq, the director of West Sumatra traffic police.
He said police were still investigating the cause of the accident, but survivors told authorities that the driver lost control of the vehicle in an area with a number of steep hills in Padang after the brakes failed.
The 12 bodies, including those of two children, were mostly pinned under the overturned bus, Sidiq said.
All the victims, including 23 injured people, were taken to two nearby hospitals, he said.
Thirteen of the injured were treated for serious injuries, Sidiq said.
The driver was among those in critical condition.
Television footage showed the mangled bus on its side, surrounded by rescuers from the National Search and Rescue Agency, police and passers-by as ambulances evacuated the injured victims and the dead.
Road accidents are common in Indonesia because of poor safety standards and infrastructure.
In 2024, a bus carrying 61 students and teachers returning from an outing to a high school in Depok, just outside Jakarta, slammed into cars and motorbikes after its brakes failed, killing 11 students and injuring dozens of others.
In 2023, a tourist bus with an apparently drowsy driver slammed into a billboard on a highway in East Java, killing at least 14 people and injuring 19 others.
