An Indian court has sentenced a police volunteer to life in prison after finding him guilty in the rape and killing of a trainee doctor last year.

An Indian court awarded a life sentence on Monday to a police volunteer convicted of the rape and murder of a junior doctor at the hospital where she worked in the eastern city of Kolkata, rejecting demands for the death penalty and saying it was not a rare crime.

India's West Bengal state launched an appeal on Tuesday to overturn a life sentence in favour of the death penalty for a police volunteer convicted of the rape and murder of a junior doctor, a senior lawyer aware of the development said.

The police had sought the death penalty in a horrific but familiar crime. Its handling by the local authorities had outraged the nation.