Karunanidhi's life will be in danger on December 6, if the recruitment age of police personnel was not increased to 29 years, the card said. It was received on Tuesday evening by Municipal Chairman K Selvaraj.
The date and place seal was not clear on the card, they said.
The minimum recruitment age for police personnel in the state is 24 years. Tirupur police registered a case and investigations are on.
Unconfirmed reports said that the card carried the name of a fundamentalist group.