"We have received a letter from IB about a threat from a terrorist outfit ahead of Aug 15," a source said.
"Delhi has been put on high alert and security has been beefed up in markets, crowded places, high rise buildings and government offices following the alert," the source said.
According to police sources, IB alerted Delhi Police for the possible terror strike in the national capital after a speech delivered by Hafiz Mohammad Saeed, chief of the banned group Jamaat-ud-Dawa, a few months ago in Karachi calling for jihad.
"Saeed called for jihad across India and told LeT to strike in Delhi," said a source.
"Intelligence agencies quoted Saeed's speech in its alert to Delhi Police," said another source.
Saeed is alleged to be responsible for the 2008 Mumbai attacks.
"All the neighbouring states have also been alerted and security has been beefed up at border check posts, railway stations, metro and all interstate bus terminus to avoid any unprecedented activity," said a senior Delhi Police officer.