India restarted city metro services Monday after a nearly six-month hiatus as the vast nation overtook Brazil to become the second-worst affected country in terms of coronavirus cases, behind only the United States.

Despite warnings that it could eventually overtake the US, India has been steadily re-opening its pandemic-battered economy.

Masked commuters sat in mostly empty coaches and flashed victory signs to journalists early Monday 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 facemasks and sanitising," passenger Deepak Kumar, who took the subway to work in Delhi, told AFP.

"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 resumption of metro services came as the South Asian nation recorded 4.2 million infections since the pandemic began, health ministry data showed.

It surpassed Brazil's total, making it the second-highest tally behind the United States' 6.25 million.

India has also recorded 71,642 deaths, fewer than the 126,203 in Brazil and 188,540 in the US.

Many experts, however, say it is not testing enough people and not properly recording many deaths, meaning the real numbers may be much higher.

'Alarming'

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.

On Monday it reported an increase of more than 90,000 cases.

Its caseload had moved past four million on Saturday, only 13 days after hitting three million.

Delhi, a city teeming with 20 million people, recorded 3,256 new infections on Sunday -- its highest single-day spike in 73 days. It was also the first time cases crossed the 3,000 mark during this period.

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 percent increase over two weeks or about two percent per day," Jameel told AFP.

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, on Friday revised the testing criteria, allowing anyone to get a test without a doctor's letter.

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.

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 percent in April to June.