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 large number of personnel who guard the chief minister's private residence, and spend a night at the house without anyone noticing.

The man scaled a wall of Banerjee's house at 34B Harish Chatterjee Street to enter around 1 am, 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 said.

"They informed the Kalighat Police Station who arrested the man," he told PTI.

Police said it is still unclear why he trespassed into the house, and going by the appearance, "he seems 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.