Authorities said they arrested Moyna Chowkidar, 38, on the outskirts of the northeastern city of Sylhet on Tuesday night, after receiving a tip-off from local residents.
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Television footage showed hundreds of people celebrating and shouting "hang him!" as a handcuffed Moyna was taken away in a police van.
"Moyna is the prime accused in the murder of Samiul (Alam Rajon). He caught the boy and brought him to other culprits," local police chief Akhter Hossain told AFP.
He is the seventh person to be arrested over the July 8 killing of the boy, who was tied to a pole and then subjected to a brutal assault in which he pleaded for his life.
One arrest was made in Saudi Arabia after officials received a tip from members of the large Bangladeshi expat community in the country.
The 28-minute video of Samiul, which was widely circulated after being posted on social media, has prompted deep soul-searching among Bangladeshis as well as a series of protests.
Thousands of people demonstrated in Samiul's home city of Sylhet on Tuesday and in a dozen other cities and towns including in the capital demanding that the killers be sent to the gallows.
Samiul was accused by his attackers of stealing a bicycle, although his family says he was innocent.
Police said their investigation had found "no evidence of theft". "The accusation was baseless. He did not steal anything," Hossain said.
In the video, the terrified youngster can be heard screaming in pain and repeating: "Please don't beat me like this, I will die."
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At one stage he is told to walk away. But as he tries to get to his feet, one of the attackers shouts: "His bones are okay. Beat him some more."
An autopsy found 64 separate injuries had been inflicted on the teenager.
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