Prayers have been said in churches across India for an elderly nun gang-raped at a convent in an attack that has intensified anger over sexual violence.

The assault on the 71-year-old is the latest in a string of high-profile rapes in India and comes amid a spate of attacks on churches that prompted Hindu nationalist Prime Minister Narendra Modi to promise a crackdown on religious violence.

The nun was attacked late on Friday after a gang of half a dozen robbers broke into a convent school in eastern West Bengal state and ransacked the premises, police said.

The robbers gagged a security guard before assaulting the nun. They then entered the principal's room and stole cash, a laptop and a mobile phone.

Four of the six attackers have been identified through CCTV footage and a reward of 100,000 rupees is on offer for any leads.

Eight others have been detained for questioning but a breakthrough has proved elusive.
Arnab Ghosh, a police superintendent who visited the convent near the town of Ranaghat, said the robbery appeared carefully planned.

"CCTV footage showed six men, aged between 20 and 30, scaled the boundary wall about 11.40 pm and entered the school and disconnected the telephone lines," he said.

"At least two of them were armed and the rest were carrying burglary tools. In the chapel, a holy scripture was found torn and ... a bust of Jesus was broken," Ghosh said.

Prayers were held on Sunday in churches in West Bengal for the nun, who is recuperating at a hospital in Ranaghat, about 70 kilometres from the state capital Kolkata.

"In our Sunday Mass, we prayed for the sister to recover quickly from trauma, fear and her physical injuries," archbishop Thomas D'Souza said.

"They not only committed a heinous crime they also vandalised the chapel ... This is the first time such an attack has happened in India."

Christian leaders in Kolkata said they were planning a candlelight vigil on Monday followed by a solidarity rally in support of the victim.