Pranab Mukherjee, who served as the president of India from 2012 - 2017, is in critical care and on a ventilator, one day after he confirmed that he had tested positive for Covid-19.
The 84-year-old was in hospital for a surgical procedure to remove a blood clot from his brain when he discovered he had the virus.
Local media have quoted hospital sources as saying that his condition remains "critical", and that the next few hours will be important.
Apart from serving as president, Mukherjee has held every important portfolio - finance, defence and foreign affairs - in government.
He also served on the boards of the International Monetary Fund and the World Bank.
India has the world's third-largest caseload of the virus, though death rates are still relatively low.
However, the speed at which cases are increasing has worried experts.
The country took just 20 days to go from one million to two million cases.
In the last 24 hours the country has added nearly 64,000 cases, taking the overall tally to nearly 2.