In the national capital, over 25,000 police personnel, including those from paramilitary forces and sharpshooters from the National Security Guard, have been deployed in the run-up to the celebrations, which will be attended by Yingluck Shinawatra, the first woman prime minister of Thailand.
Security has been similarly stepped up in the metropolises and sensitive states like Jammu and Kashmir and the North East.
As part of the stepped up measures, the airspace over the capital will remain closed for an hour tomorrow from 11.15 am to 12.15 pm.
Snipers have been deployed at high-rise buildings while over 160 CCTVs are keeping a tight vigil on people's movement between Rajpath and Red Fort, the route of the parade which showcases India's military might and cultural diversity.
Mobile hit teams, anti-aircraft guns and sharpshooters of the NSG are also on the job at various places while paramilitary and Delhi police commandos are keeping a close watch along the route of the eight-kilometre-long Republic Day parade from Raisina Hills to Red Fort.
A multi-layered security ring has already been put in place at Rajpath, where President Pratibha Patil will unfurl the tricolour and take the salute of marching contingents.
Besides the air defence measures, helicopters of the Indian Air Force will hover around Rajpath and all along the route of the parade.
Patrolling in crowded market areas has been intensified and checking and frisking in the Metro, railway stations and bus terminals tightened. Security at the IGI airport has also been strengthened in coordination with the Central Industrial Security Force.
Checking and frisking have also been intensified at all entry points with police setting up barricades to keep a vigil on all those entering the capital.
Security has been tightened across Kashmir by authorities to frustrate any attempt by militants to disrupt peace.
Police and paramilitary forces have been deployed in strength at Bakshi Stadium, the main venue of the Republic Day celebrations in summer capital Srinagar.
Vital installations in the city are being guarded, the officials said adding that random frisking, checking of vehicles and area domination have been intensified.
In Mumbai, ahead of the celebrations tomorrow, nearly 40,000 police personnel have spread out across the metropolis to maintain a tight vigil.
Quick Response Teams carrying hi-end weapons have been deployed at strategic locations, the police said.
The Railway Police Force will carry out random checks and frisk passengers at the stations.
A parade by Maharashtra police will be held at Shivaji Park in the presence of Governor K Sankaranarayanan, Chief Minister Prithviraj Chavan, his deputy Ajit Pawar and DGP K Subramaniam, among others.
Vehicular movement on seven roads near Shivaji Park in Dadar area will remain closed for six hours beginning from 6 am tomorrow, the police said.
Security has been tightened in Assam with a special focus around major installations like ONGC and OIL.
Assam Chief Secretary N K Das chaired a meeting to review the security measures and other arrangements initiated to ensure the smooth conduct of the event, official sources said in Guwahati.
Security has been beefed up around important installations like Oil India Limited, ONGC and Indian Oil Corporation, besides at railways, airport, bus depots and other public places, sources said.
The public, in general, have been requested to be on high alert against subversive activities by insurgent outfits.
At least 12 outfits including the anti-talk faction of ULFA, Coordination Committee (umbrella organisation of seven militant outfits of Manipur), Kamatapur Liberation Organisation, National Liberation Front of Trip's, Tripper Peoples' Democratic Front and Hundreder National Liberation Front have jointly called for a 'boycott' of the Republic Day celebrations and a general `shut-down' on January 26.