The Air India [Images] Office in Chennai has received a letter warning attacks by 'suicide bombers' and threats to disrupt the Pravasi Bharatiya Divas (PBD) beginning in the city on Thursday, police said.

The letter said terrorists belonging to Lashkar-e-Taiba, Jaish-e-Mohammed and Jamath Ud Dawa had infiltrated Chennai and would disrupt the PBD in which President Pratibha Patil [Images] and Prime Minister Manmohan Singh [Images] are participating, police said.

Suicide bombers of the outfits would strike during the conference, the letter said.

The letter had been posted from Karaikal. Police officials from Chennai Commissionerate and Karaikal are investigating the terror threat after the letter written in Tamil was received at the Air India office here yesterday, police said.

Security has already been tightened since last week following an e-mail terror threat sent to the Confederation of Indian Industry(CII), one of the organisers of the PBD, by the Indian Mujahideen [Images], police said.