Nobody was injured, they said, adding, some furniture and air-conditioner in the building were damaged in the incident.
Ramanidharanm, who represents Anthiyur constituency, was not in the office at the time of the incident.
Police, quoting eyewitness, said some unidentified persons came in motorcycles to the office and after hurling the bomb they fled.
Police said it was suspected that activists of Pattali Makkal Katchi (PMK) might be behind the incident as there were widespread protests in various places of Tamil Nadu in the wake of the arrest of party founder leader S Ramadoss and his son and former Union minister Anbumani following the violence at Marakkanam in Villupuram district which was allegedly targeted against Dalits on April 25.
Ramadoss had been arrested on April 30 after he failed to wind up a PMK function which was blamed by Chief Minister J Jayalalithaa as the reason for the violence.
Anbumani was arrested on Friday from Chennai in connection with a case filed last year under different sections including Unlawful Assembly, Activities prejudicial to Peace, Violation of Police Rules.
Meanwhile, stones were pelted at a Tamil Nadu Transport Corporation bus at Chavadipalayam, 12 km from here last night damaging its windscreen.