Police sources said the case was registered on the directions of the DGP, who had received several complaints in this regard.
The case was registered at East Police station under Section 228(A) of IPC, as per which two years imprisonment and a fine can be imposed for the offence.
Opposition leader Ramesh Chennithala had recently written to Chief Minister Pinarayi Vijayan, demanding that a case be registered against Radhakrishnan, a former minister and Speaker of the Assembly, as he had "deliberately" named the victim.
A police officer who allegedly posed indecent questions when the victim had approached him to register her complaint, was suspended yesterday.
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The victim had recently gone public about the trauma she underwent two years ago and had said she was being haunted by by the four persons even now.
CPI(M) General Secretary Sitaram Yechury had come out against Radhkrishnan, stating that he should not have revealed the victim's name.
CPI(M) Politburo member Brinda Karat had said it was a 'mistake' to name the victim.