The protesters placed the body of the deceased, identified as Rajan, on the highway and raised slogans against the Delhi Police.
The angry protesters pelted police with stones. However, no casualties were reported and no case of rioting has been registered, said a police official.
The situation was brought under control after senior officials intervened and assured of justice, the official added.
The 19-year-old was said to be thrashed by constable Pramod Kumar in outer Delhi's Narela area at a Dussehra fair on October 21.
"The constable has been placed under suspension and an inquiry initiated. Further action shall be taken accordingly," Deputy Commissioner of Police (Outer) Vikramjit Singh told IANS.
An argument broke out between Rajan and a person manning a ticket counter at the fair after the latter alleged that he had given a fake Rs.1,000 note to buy a ticket for a joyride, police said.
Pramod, who was deployed at the fair, intervened and allegedly beat up the man, said a police official.
When he collapsed, a police team rushed him to Raja Harish Chandra Hospital, from where he was referred to Lok Nayak Jai Prakash Narayan Hospital. He died in the early hours of Monday.
"A preliminary medical report revealed the young man had extremely high blood sugar and he went into a diabetic coma, which is life-threatening," said the official, adding that the constable's role would be probed and action taken if charges were found to be correct.