The juvenile, said to be around 16-year-old, was caught from the spot by locals after the victim raised an alarm and later handed over to the police.
"The girl, who is pursuing a beautician's course in Lajpat Nagar, was going to the bus stop at 11 AM when the boy, who lives in the neighbourhood, passed lewd comments at her.
When the girl objected, he groped her from behind, said a senior police official.
As the girl raised an alarm, locals nabbed the youth, thrashed him and handed him to the police.
Police has registered a case of outraging the modesty of a woman under Indian Penal Code and Scheduled Caste and Scheduled Tribe (Prevention of Atrocities) Act at Sunlight Colony police station.
The youth was produced before the Juvenile Justice Board.
His family has been asked to provide his school certificates to determine his exact age, police said.
