Hundreds of protesters took to the streets of the city of Khulna to demand justice for Mohammad Rakib, who died late Monday after the employer forced the compressor tube into the boy's rectum and turned on the machine.
"This is the worst murder I have ever seen in my professional life. This is simply medieval barbarism," Nibash Chandra Majhi, police commissioner of the southwestern city, told AFP.
Residents demonstrated after earlier storming the motor mechanic's workshop where the killing occurred, dragging out the boss and two others and handing them over to police.
The killing comes after nationwide protests in July over the lynching of another 13-year-old boy, which was captured on video and uploaded onto social media.
Police have arrested about a dozen men over the murder of Samiul Alam Rajon, who was tied to a pole and beaten to death while pleading for his life in the city of Sylhet.
The video went viral, prompting soul-searching over the nation's treatment of poor children.
In the latest incident, police said the employer, Mohammad Sharif, was enraged after Rakib left his workshop a few months earlier for another job.
"The boy was dragged inside the workshop and the culprits led by Sharif inserted a blow pipe of an air compressor, which is used for inflating car tyres, in his rectum and switched on the machine," he told AFP.
He was rescued by locals and rushed to hospital but died of internal injuries.
