However, police said the youth, identified as Vivek Rana alias Vicky, was an epileptic patient who died at around 1:30 AM.
DCP P C Baranda told reporters that an inquiry has been ordered to look into the allegations of torture.
Vicky was one of the 18 youngsters identified and arrested by city police for allegedly attacking shops, vehicles and many commuters in Maninagar area on November 8.
As per the complaint filed in Maninagar police last week, Vicky and others indiscriminately attacked more than 20 shops and vehicles to instill fear among locals and establish their dominance in that area.
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While police arrested 18 accused over last one week in this connection, the Crime Branch arrested Vicky on Friday morning and handed him over to Maninagar police by evening on the same day.
Vicky's sister Renuka today alleged that her brother was severely beaten up by Maninagar police when he was in their custody.
Police said Vicky was epileptic and was admitted to hospital after he developed symptoms.
"As soon as he sought medical help due to his epilepsy problem, police took him to L G Hospital on Saturday morning at around 6. After 18 hours, he died at around 1:30 am today," said Baranda.
The officer said an inquiry has been ordered to find out the truth behind the allegations of torture.