India on Sunday set a coronavirus record when it reported 78,761 new infections in 24 hours, the world's highest single-day rise, according to its health ministry figures.

Home to 1.3 billion people, India is already the world's third-most infected nation with more than 3.5 million cases, following the U.S. (5,96 million cases) and Brazil  (3.84 million cases).

The COVID-19 death toll in India jumped by 948 to 63,498, the federal health ministry data showed.

In the regular monthly radio address on Sunday, India's Prime Minister Narendra Modi did not comment on the milestone but called on Indians to observe health safety measures.

"It is important that every citizen is healthy and happy and we defeat coronavirus completely together," Modi said in Hindi.

"Corona(virus) can only be defeated when you remain safe, when you fulfill the resolve of keeping a safe distance of two yards and wearing masks."

Experts warn that while a ramp up in testing in recent months was encouraging, more needed to be done to capture the scale of the pandemic in the world's second-most populous nation.

The virus has badly hit megacities such as financial hub Mumbai and the capital New Delhi, but is now also surging in smaller cities and rural areas.

Low testing rates

"Testing per million in India at 30,000 remains the second lowest in top 10 (virus-infected) countries. Mexico is lowest at about 10,000," virologist Shahid Jameel, who heads the Wellcome Trust/DBT India Alliance said.

"We are still finding one positive in every 11 to 12 tests. This number should be one in 20 simply and means that testing is still sub-optimal and the outbreak is growing."

Doctor Hemant Shewade, a Bangalore-based community medicine expert, added that India also needed to focus on reducing deaths – the fourth-highest in the world according to a tally by Johns Hopkins University.

The government collects fatality numbers from positive cases but not from suspected infections, raising concerns among scientists that the true picture of the epidemic is not being reflected in the official toll, he said.

Despite the surging case numbers, Prime Minister Narendra Modi has been pushing for a return to normalcy to lessen the economic pain of the pandemic, having earlier imposed strict lockdowns of the country's 1.3 billion people.

The federal home ministry has decided to let underground train networks reopen with some restrictions in New Delhi, India's capital of about 20 million people.

The subway will start running on September 7 for the first time since March, when India imposed the world's strictest lockdown to contain the spread of the new coronavirus.

The Home Affairs Ministry said gatherings of up to 100 people would be allowed with face masks and social distancing at cultural, entertainment, sports and political events from next month.

(With input from agencies)