A man sneaked into Chief Minister Mamata Banerjee's residence in south Kolkata's Kalighat area in the early hours of Sunday, police said.
The incident triggered a security scare with questions being raised that how he went past the Z-Plus security cover that the chief minister gets, and enter her private residence in the 'highly secure' neighbourhood to spend a night without anyone noticing.
The man scaled a wall of Banerjee's house at 34B Harish Chatterjee Street to enter around 1 am, police said.
"An incident of security breach was reported at the residence of the chief minister. A person somehow sneaked into the CM's residence complex with mischievous intention," police said.
He remained inside the house, spending the night sitting at a corner, and was spotted only in the morning by security personnel, a senior police officer told PTI.
"They informed the Kalighat police station, which arrested the man," he added.
Police said that going by the appearance of the man, "he seemed to be somewhat mentally unstable".
"We are talking to him and trying to find out his whereabouts. We are also trying to find out whether someone instructed him to enter the CM's residence with a motive. An investigation is underway," the officer said.
Police said immediate steps are being taken to enhance security in and around the CM's residence.
Long iron bars have been put up at the Kalighat bridge near Banerjee's residence to keep a check on any unwanted movement, they said.
The chief minister of West Bengal does not have an official residence, and Banerjee's house is located in one of the oldest neighbourhoods of Kolkata, near the centuries-old Kalighat temple, on the bank of a canal -- Adi Ganga.