Condemning the letter threatening to kill the 12 persons, state BJP President Pon Radhakrishnan said in a statement here that it was a threat to the state government and a 'challenge' to communal harmony.
"We are seeing the letter sent to BJP office as a threatening letter to Tamil Nadu Government to the police force and as a challenge to communal harmony."
He sought strengthening of security to party's leaders in the state and additional security to leaders, who have now been threatened, arrest of those behind the letter.
The letter, received yesterday, had stated that the 12 leaders would be killed if their demand for immediate release of Kerala-based PDP leader Abdul Nassar Madani, an accused in the 2008 Bangalore serial bomb blasts, was not met immediately.
The letter came in the background of recent murder of BJP state General Secretary V Ramesh in Salem and Hindu Munnani leader S Vellaiyappan in Vellore this month.