Indian police have detained eight men for questioning over the gang-rape of an elderly nun as she tried to stop burglars ransacking a convent school.

The 71-year-old nun was attacked by 12 robbers who escaped with cash, a mobile phone, a laptop and a camera intended for pupils in Ranaghat town .

The crime has heightened fears for the Christian minority in the world's second most populous nation highlighting sexual violence once again in the nation.

Six suspects were caught on CCTV released by police and it is believed that four men identified from the footage were among the group arrested on March 15, The Independent newspaper reported.

They are being held in the Nadia district of West Bengal, the Times of India reported, and investigators are investigating motives including extortion and revenge as well as robbery.

CCTV showed robbers aged between 20 and 30 clamber over the convent's wall at near midnight on March 13before cutting telephone lines.

Investigators said the nun tried to stop the burglars from taking away the valuables, but the suspects attacked and raped her instead. Two other persons who were inside the facility during the attack were injured during the burglary, police said.

A security camera in the school managed to record the suspects passing through a corridor, enabling authorities to have a clear look at their faces. But so far, investigators have yet to ascertain their identities.

At least eight individuals have been brought in for questioning, according to Nadia superintendent Arnab Ghosh. He declined to elaborate.

"CCTV footage showed that six men, aged between 20 and 30, scaled the boundary wall around 11:40pm and entered the school and disconnected the telephone lines," he was quoted as saying in a report by Agence France-Presse.

"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 added.

The nun was safe March 16 and recovering from the attack after undergoing surgery.

Kolkata Archbishop Thomas D'Souza noted that Christians living in the area have every right to feel insecure given the lack of security facilities there. He noted that the closest police station near the convent was around six kilometers (four miles) away.

Asked about the recent attack, the archbishop explained he could not say whether the latest episode highlighting the dangers Christians faced in the country had anything to do with related incidents in Delhi and Bangalore recently.

"Anything is possible. Nothing can be ruled out, but until police make some arrests, nothing can be said for sure as well," the archbishop said.

On Sunday, churches across India offered prayers for the nun, demanding that the government act swiftly on the issue, just as in the other acts of violence committed against minorities living in the country.