Eight monks died in Thursday's accident, local police say. Another dozen were injured, several of them seriously.
The suspect is a child, police chief Pairoj Thaiphutsa told reporters.
According to police, the 11-year-old, who had taken his parents' car without permission, lost control of the vehicle and crashed into a procession of monks on a pilgrimage. The boy is now in custody.
The accident occurred in the Thai province of Mukdahan, near the border with Laos.
