The central security agencies had given inputs to police that Pakistan-based Lashker-e-Taiba terror group may attempt to attack markets or places of strategic importance in the city.
A ground-to-air security apparatus has been put in place especially near the Red Fort from where Prime Minister Manmohan Singh will unfurl the tricolour and address the nation on Thursday.
Thousands of Delhi Police and paramilitary personnel have been deployed at market places, airport, railway stations, inter-state bus terminals, metro stations and places of strategic importance.
The alert by central security agencies quoted a paragraph from speech of Hafiz Saeed, founder of Lashker-e-Taiba, delivered last month in Pakistan where he had openly threatened that his terrorists will carry out an attack in the national capital.
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Around 50 CCTVs have been installed in and around the Red Fort to ensure incident-free celebrations while sharpshooters of the NSG will be deployed on the high-rises near the 17th century Mughal monument, officials said.
At least 80 companies of security forces (around 6,000 personnel) will be deployed at the Red Fort and the surrounding areas during the Independence Day celebrations, said a senior police official.