The celebrations on August 15 - which mark India's independence from British rule in 1947 - have been frequently disturbed by insurgent groups who use the national day to mount attacks and underline their presence, forcing heightened vigil.
But the July 11 train bombings in the country's financial hub, which killed 186 people, a foiled plot to bomb trans-Atlantic flights last week, and a US embassy warning of a likely al Qaeda attack in Mumbai or Delhi, have unnerved security agencies this year.
"Intelligence inputs ... have indicated renewed determination of terrorist outfits to undertake terrorist acts, target prominent political leaders and symbols of national importance," said Ajay Chaddha, a top Delhi Police officer.
Thousands of policemen and troops were positioned across cities and sites considered to be targets of Islamist militants fighting Indian rule in Kashmir as well as insurgent groups in the troubled northeast, officials said.
Snipers, metal detectors, x-ray machines, sniffer dogs, radio frequency jammers and security cameras were also being deployed and troops armed with machine guns were patrolling the airport in New Delhi, officials and witnesses said.
The threat is considered to be the highest in the capital as the main celebrations on August 15 are centred in the city with the prime minister hoisting the national flag at a historic fort and addressing the nation from its ramparts.
Threat to nuclear facility
Following last month's bombings, Mumbai, too, has been placed on high alert and a security blanket thrown around key government and private buildings, dams, bridges and a key nuclear facility in the region, officials said.
The train bombings in the financial
capital of Mumbai killed 186 people
"Several security agencies have spoken of a threat perception to many places including BARC," said D.K. Sankaran, the top bureaucrat of Maharashtra state, of which Mumbai is the capital, referring to the Bhabha Atomic Research Centre.
"We have taken adequate measures to guard our nuclear facility," he added.
Srinagar, the summer capital of Indian Kashmir, resembled a city under siege with police and soldiers in battle-gear patrolling the streets, frisking people at bus stations and marketplaces, residents said.
The city is the centre of a nearly 17-year-old revolt against Indian rule in which tens of thousands of people have died.
Threat from the northeast
Intelligence officials in the country's northeast, home to nearly two dozen insurgencies, said heavily armed guerrillas had sneaked into the region in small groups from their hideouts in neighbouring Bangladesh and Myanmar to create trouble.
"But this time the agencies seem to have sophisticated information for the past three weeks that some terrorist groups are in Delhi"
Ajai Sahni, head of the New Delhi-based Institute for Conflict Management
Police in over half-a-dozen states spread across eastern, central and southern India have also increased vigil to prevent attacks by Maoist rebels who say they are fighting for the rights of poor farmers and landless labourers.
India had been on the map of al Qaeda for several years now and this year's threat perception was not very different from the previous ones, said Ajai Sahni, head of the New Delhi-based Institute for Conflict Management.
"But this time the agencies seem to have sophisticated information for the past three weeks that some terrorist groups are in Delhi," he said.