Checkpoints were set up to search vehicles on access roads to the capital ahead of the celebration which marks the date when independent India's constitution came into effect.
"There is no specific alert from intelligence agencies but we are on high alert anyway"
Mumbai Police Commissioner
Lee Myung-bak, the South Korean president, in India on a four-day state visit, is attending the New Delhi parade.
The display is a showcase of the Indian military's latest hardware, acquired as part of a massive modernisation drive costing tens of billions of dollars.
Rail stations, power and communications hubs and other key landmarks across the country have been placed under increased security for the holiday, while naval and coast guard forces have also been placed on alert.
In the financial capital, Mumbai, some 42,000 police have been placed at key locations across the city.
"There is no specific alert from intelligence agencies but we are on high alert anyway," D Sivanandan, the Mumbai Police Commissioner, told Reuters.
Security concerns have even seen federal troops deployed at the Taj Mahal memorial in Agra, the city's police chief said.
The security crackdown comes after Indian authorities put airports across the country on high alert last week, following western intelligence tip-offs that al-Qaeda-linked fighters planned to hijack an aeroplane.
|Security has been put on high alert following the spate of attacks in recent weeks [EPA]
Indian media have also cited government intelligence reports about possible threats from the Pakistan-based Lashkar-e-Taiba group, following reports that it had purchased 50 sets of paragliding equipment which could be used to launch airborne attacks.
The raid by Taliban fighters and suicide bombers in the centre of the Afghan capital, Kabul, last week also set alarm bells ringing, with its strong echoes of the 2008 attacks in Mumbai.
On Monday, security officials in Kashmir said they had uncovered a large cache of arms and explosives which they believe was meant for a Republic Day attack.
Several separatist groups in the country's restive northeast and in the Indian-controlled part of Kashmir have called general strikes for Republic Day.
On Sunday, suspected separatists blew up a railroad track in India's remote northeastern Assam state, but no one was hurt, police said.
The blast damaged the railway track near Deka Mising Gaon in northern Assam, said Bhaskar Mahanta, the state inspector-general of police.