A security cordon now surrounds most major Indian airports
Indian authorities have put three major airports on high alert after a threat of possible militant attacks.
The alert was prompted by an email purportedly from the Deccan Mujahadeen, the previously unknown group which claimed the Mumbai attacks.
The latest email threatened an attack on the air transport system - security has been tightened at airports in Delhi, Madras (Chennai) and Bangalore.
At least 188 people were killed in attacks over several days in Mumbai.
India says the attackers had links to Pakistan. It denies any role and has cast doubt on the allegations.
Indian authorities say extra forces have been deployed at the airports in Delhi, Madras and Bangalore.
Passengers are being asked to arrive at the airport three hours before their flight, reports say.
They are being thoroughly searched and all their bags are being opened and checked by the security forces, local television channels reported.
Every vehicle is being checked before being allowed in through the security cordon.