Central security agencies have given inputs to police that Pakistan-based terror outfit Lashkar-e-Taiba may try to target markets or places of strategic importance in the city.
A ground-to-air security system has been put in place near the Red Fort where Prime Minister Manmohan Singh will unfurl the Tricolour and make his address to the nation on Thursday.
Thousands of Delhi police and paramilitary personnel have been deployed at the airport and at market places, railway stations, inter-state bus terminals and metro stations.
Besides patrolling of the skies by helicopters, an air defence mechanism will also be put in place around Red Fort.
The alert by central security agencies quoted a paragraph from a speech by LeT founder Hafiz Saeed last month in Pakistan in which he openly threatens that his terrorists will carry out an attack in the national capital.
Hundreds of CCTVs have been installed in and around Red Fort with sharpshooters from NSG to be deployed on high-rises near the 17th century Mughal monument, officials said.
