Police said the 17-year old boy of R N Kottai suffered injuries in the assault by the public today and had been admitted to the Government hospital here.
The girl, daughter of a mason, has also been hospitalised and the condition of both was stated to be serious, doctors said.
The boy had taken the girl, his neighbour, to a secluded place by luring her with a chocolate and sexually assaulted her yesterday.
As the girl started bleeding, her parents rushed her to a hospital where she told doctors about the incident.