A nun in her 70s was gang-raped by a group of raiders when she tried to prevent them from robbing a Christian missionary school in eastern India, police said.

The nun was admitted to hospital in a serious condition after being attacked by seven or eight men at the Convent of Jesus and Mary School in Nadia district, 80 kilometres north east of the West Bengal state capital of Kolkata.

Police are searching for the men who escaped after the attack.

The robbers tied up the school's security guards with ropes and entered the nuns' room, where the women were sleeping. They took one of the nuns to another room when she tried to block their way and then raped her, a police officer said.

The woman who was attacked is either 71 or 72 and is the oldest nun at the school, he said.

The men escaped with some cash, a mobile phone, a laptop computer and a camera, all belonging to the school. They also ransacked the school's chapel and holy items, the Press Trust of India news agency cited the archbishop of Kolkata, Thomas D'Souza, as saying.

Scores of angry students, their parents and teachers blocked a nearby road and rail tracks for several hours demanding swift police action.

Mamta Banerjee, the state's top elected official, strongly condemned the attack and ordered a high-level police investigation. Archbishop D'Souza appealed to people to maintain peace and harmony in the area.

India has a long history of tolerance for sexual violence but the December 2012 fatal gang rape of a 23-year-old woman aboard a moving bus in New Delhi caused outrage across the nation.

The outcry led the federal government to rush through legislation doubling prison terms for rapists to 20 years and criminalising voyeurism, stalking and the trafficking of women. The law also makes it a crime for officers to refuse to open cases when complaints are made.