Here are 10 developments in this story:
The Indian Air Force has said the flypast, a highlight of the parade, is still on, after concern that it might be cancelled due to bad weather. It has rained in Delhi all morning.
The US president has now joined President Pranab Mukherjee and Prime Minister Narendra Modi at Rajpath, the wide avenue in the heart of Delhi where India's 66th Republic Day celebrations will take place in a short while from now.
There is unprecedented security, with a seven-layer ring of 50,000 security personnel, including 500 US Secret Service Agents, deployed. The Delhi Police, said sources, has been asked to check people wearing black caps and mufflers, after intelligence that Maoists might try to stage protests.
At Rajpath, there are 500 police officials trained in detecting suspicious movements in the public and among participants. They have been specifically trained to profile potential terrorists.
The highlight of this year's parade will be the indigenously developed surface-to-air Akash medium range missiles and weapon locating radar. The long-range advanced MiG-29K fighter will be seen for the first time.
Three all-women marching contingents of the Army, Navy and the Air Force represent the theme of the parade, "Nari Shakti (woman power)". Also, a display on the PM's Beti Bachao, Beti Padhao (Save the daughter and educate her) campaign will lead the Republic Day floats.
In the afternoon, the US President is expected to meet Congress leaders Sonia Gandhi, Rahul Gandhi and former prime minister Manmohan Singh, with whom he is said to share a special rapport. He will later attend President Pranab Mukherjee's "At-home."
At 5.30 pm, the US President will attend the India-US CEO Forum meeting at the hotel Taj Palace. Another meeting with business delegations from the two countries - India-US Business Summit - will take place later in the evening.
Mr Obama, the first US president to be chief guest at the Republic Day celebrations, arrived with his wife Michelle on a three-day visit yesterday. PM Modi broke protocol and received President Obama at the airport, greeting him with a hug.
A day of bonhomie between President Obama and PM Modi produced the most significant outcome of the visit; an agreement on the nuclear deal, which had been stalled for over six years.
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