NEW DELHI (BLOOMBERG, AFP) - India has the world's second-highest number of coronavirus cases, passing Brazil on Monday (Sept 7) to reach more than 4.2 million confirmed infections as the epidemic surges across the South Asian nation.

Now the global virus hot spot, India added the largest number of cases in a day with 90,802 recorded overnight on Sunday, according to data released by India's health ministry.

More than 71,000 people have died from Covid-19, making India the third-largest by number of deaths.

Prime Minister Narendra Modi's government initiated the world's biggest virus lockdown in the country of 1.3 million people in late March, but the economic and social costs forced a gradual reopening even as infections surged at a record pace.

India's economy posted the biggest contraction among major economies last quarter, with its gross domestic product shrinking 23.9 per cent in the three months to June from a year earlier.

On the same day that India surpassed Brazil's infection tally, the country's largest metro system in the capital New Delhi began operating again - after a nearly six-month hiatus - while limited religious and sporting events have been allowed to restart and bars are set to open.

Masked commuters sat in mostly empty coaches and flashed victory signs to journalists at metro stations in the capital New Delhi - one of the most badly hit cities along with financial hub Mumbai.

Passengers can only sit on alternate seats, and after undergoing thermal screening.

"There was no problems commuting in the metro. Social distancing is being followed and...people are using face masks and sanitising," said passenger Deepak Kumar, who took the subway to work in Delhi.

"For our lives to move on, we have to get out of our homes...so this is a good move by the government to start the metro trains."

The re-opening of subway services was part of an effort by the government to gradually restart economic activity after the strict lockdown imposed from late March saw growth contract by a record 23.9 per cent in April to June.

Since August, the country of 1.3 billion people, home to some of the world's most densely populated cities, has been reporting the highest single-day rises in the world.

Virologist Shahid Jameel, who heads the Wellcome Trust/DBT India Alliance, said the key factor to watch is the growth rate in infections, which he called "quite alarming".

"Over the past two weeks, the...average has moved from about 65,000 cases per day to about 83,000 cases per day, that is about a 27 per cent increase over two weeks or about 2 per cent per day," Dr Jameel said.

India has been testing more than 10 million people per day on average, with plans to ramp it up further.

The Indian Council of Medical Research, the scientific agency leading the government's response, last Friday revised the testing criteria, allowing anyone to get a test without a doctor's letter.

Dr Jameel said the move was overdue.

"This will uncover more asymptomatic people, who are the real source of this expansion in India. There should also be more testing in rural districts and villages, since over two-thirds of the cases are coming from there," he said.