The national capital was put under a massive ground-to-air security apparatus for the Republic Day celebrations on Monday in the wake of the Noida shootout in which two Pakistani terrorists were killed and intelligence inputs about possible attacks.

Mobile hit teams, anti-aircraft guns and sharpshooters of the elite National Security Guards have been deployed at various locations while paramilitary and Delhi [Images] Police commandos kept a tight vigil along the route of the Republic Day parade.

Around 20,000 security personnel, including 5,500 from paramilitary forces, have been deployed across New Delhi with commandos of the Indo-Tibetan Border Patrol and Delhi Police positioned at strategic locations. Snipers were deployed at all high-rises along the parade route.

A multi-layer security ring was put in place at Rajpath, where President Pratibha Patil [Images] will unfurl the tricolour and take the salute of marching contingents.

Kazakhstan President Nursultan Nazarbayev is the chief guest at the parade, which will also be attended by Defence Minister A K Antony and the country's top political and military brass.

Prime Minister Manmohan Singh [Images] will not be attending this year's R-Day celebrations as he is recuperating after a coronary surgery.

The national capital's airspace will remain closed from 1115 IST to 1215 IST. The Notice To Airmen (NOTAM) has been issued by the air traffic controllers to the airlines.

The Uttar Pradesh [Images] ATS killed two suspected Pakistani militants in neighbouring Noida on Sunday while they were trying to enter Delhi.

Police have stepped up security measures, saying they do not want to take "any chances" ahead of the national event, especially in the backdrop of the Mumbai [Images] carnage which claimed over 170 lives.

"After Mumbai attacks, we are not taking any chances. There are inputs (about possible strikes). We are making tight arrangements for the event," a senior police officer said.

Security was stepped up at important public places and vital installations, including the Delhi Metro stations, railway stations, bus terminals and crowded marketplaces.

Electronic surveillance was increased, the officer said, adding that extra security cover has been extended to all the religious places in the capital.

Checking at borders and night patrolling was intensified in Delhi as also the barricades being put up at various interactions and canine units deployed at railway stations, the officer added.

"A special emphasis is being laid on anti-sabotage checks, access control measures and intelligence coordination. The entire route of the parade will be covered by special security and anti-terror arrangements," a senior police official said.

"Elaborate air defence measures, including deployment of anti-aircraft guns, have also been taken to check intrusion of air space," he added.