The man walked up the first floor of the CIT colony residence of Ammal, living with her daughter and DMK Rajya Sabha MP Kanimozhi, pointed the toy gun and demanded money and jewellery, DMK party sources said.
Ammal was at that time talking with a woman relative.
The 73-year-old asked the man to wait so that she could bring the money. She then rushed downstairs and called the police.
The intruder held Ammal's relative as hostage, but freed her after police arrived, and was subsequently nabbed.
When he threatened Ammal and her relative, nobody knew that the firearm was actually a toy gun. It became known only after a police enquiry.
"We have lodged a formal complaint," sources close to Kanimozhi said.
A senior police official said a probe was on and added that additional guards had been posted at Kanimozhi's residence.
Hearing about the incident from her mother, Kanimozhi rushed back home from a DMK meet.
Asked how the intruder gained entry when the house was guarded by the police, the official said the man scaled the wall from a side where there were no sentries and at a time when there was not much public movement.