The aquittal of two men accused of raping a Catholic nun in India is a "grave injustice", a senior priest in the country has said.

Archbishop of Mumbai Cardinal Oswald Gracias accused police of having a "half-hearted attitude" towards the crime, in an interview with Asia News.

Jitendra Pathak, 25, and Dinest Dhurv, 19, were released on January 5 due to lack of evidence.

They stood accused of having drugged, tied up and gang raped a 47-year-old nun at the Krist Sahaya Kendra (Christ Help Centre) in Chhattisgarh state, central India, on June 19, 2015.

A qualified nurse and a member of the Salesian Missionaries of Mary Immaculate, she was living at the centre which treats children and patients with minor injuries.

"This acquittal once again brings to our attention the problem of violence against women. It is a huge setback for all of us working for the rights and dignity of women, in particular victims of violence," said Cardinal Gracias.

"India's Catholic Church will demand justice from a higher court. We will challenge the verdict on appeal."

The attack on the nun followed the gang-rape of an elderly nun at a convent in West Bengal the previous March.

"Incidents, one after another like this, targeted against the Missionary Nuns, who have sacrificed their lives for God and for the service of humanity, cause real concern for all of us and bring shame and disgrace to the ethos of our Nation," the Catholic Bishops Conference of India said at the time.

"There is evident negligence from the part of the Governments both States' and Central, to protect the Minorities and Women in general. Even when such highly deplorable acts of violence take place, our leaders and those in authority keep silence or do not care at all to prevent such dastardly action."

Concerns have been raised by campaigners about the treatment of religious minorities in India, and persecution watchdog International Christian Concern last week condemned the "escalation of attacks on Christians" in the country.