District Magistrate P B Salim, who rushed to the spot, told PTI it was a heinous act and police were already on the lookout for the gang members.
State Urban Development Minister Firhad Hakim said the Chief Minister has ordered a CID probe into the incident.
According to authorities of the local convent school in Ranaghat sub-division, the gang broke into the convent after 12:30 am and three-four of them raped the woman after gagging her.
The nun was taken to Ranaghat Hospital by the school hostel authorities in the morning when the matter came to light.
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"We are yet to get a full picture of the incident," the DM said.
Aggrieved guardians of students began blocking the arterial NH 34 and Sealdah-Ranaghat rail tracks from 12 noon as the news spread like wildfire in the locality.
Condemning the incident Education Minister Partha Chatterjee said that the people will not tolerate this.
The culprits will be given strictest punishment, he said.
"The culprits have to be identified and arrested immediately and given exemplary punishment," he said.