India's nationwide coronavirus lockdown, the biggest in the world, will be extended until at least May 3, Prime Minister Narendra Modi has said.

The three-week lockdown of the nation of 1.3 billion people, which started at midnight on March 25, was scheduled to end at midnight on Tuesday.

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"From the economic angle, we have paid a big price," Modi said in a nationwide address. "But the lives of the people of India are far more valuable."

Address to the nation. https://t.co/26sVP2br5n — Narendra Modi (@narendramodi) April 14, 2020

Modi said some restrictions in areas further away from infection hotspots would be eased on April 20 to help poor people dependent upon daily wages.

"Till April 20, each police station, each district, each state will be monitored closely to see whether the lockdown is being followed and if that area has saved itself from the virus," he said.

"We can then decide on relaxing restrictions in those areas that are successful in this test, have successfully contained the hotspots and prevented new ones."

According to official figures, South Asian nations have so far been relatively unscathed by the pandemic, with India reporting more than 10,000 cases and 339 deaths.

But with some of the most crowded cities on the planet, there are fears that numbers could skyrocket and overwhelm shaky healthcare systems.

Some experts have also said India has not conducted enough tests and that the true number of infections is much higher.

Several states, including Maharashtra - which has the highest number of cases - Tamil Nadu and Odisha, have already announced lockdown extensions.

India's poor worst hit

But at the same time, the lockdown, with strict limits on activity, has been devastating for the economy - and for India's poor.

Millions of daily wage labourers suddenly lost their jobs, forcing hundreds of thousands to make the long trek back to their home villages, hundreds of kilometres away, often on foot.

Some died on the way, while others were shunned by locals when they made it back to their villages. One viral clip showed a group of migrants being hosed down with chemicals.

Others have been stranded in cities in cramped, unsanitary conditions where the virus could spread quickly.

New Delhi alone is providing hundreds of thousands of free meals to help those for whom the lockdown means immediate hunger.

Farmers have complained of a lack of workers to harvest crops while the grounding of thousands of trucks by the lockdown have hampered food transport.

Farms, still the bedrock of the Indian economy, are heading into their most important harvest time of the year, when many villages earn enough money to finance themselves for months to come.

The lockdown, with strict limits on activity, has been devastating for the economy [Harish Tyagi/EPA]

'Invisible assassin'

Reserve Bank of India Governor Shaktikanta Das called the coronavirus an "invisible assassin" that could wreak havoc on Asia's third-biggest economy.

The national restaurants association, which said its members employed seven million people, warned on Monday there could be "social unrest" if it did not receive financial relief.

The commerce ministry has also reportedly urged the government to consider restarting more activities "with reasonable safeguards" even if the lockdown is extended.

Even before the pandemic, the Indian economy was stuttering with unemployment at its highest in decades.

Some analysts have predicted growth could slump to 1.5-2 percent this year - well below the level needed to provide jobs for the millions coming into the labour market each month.

Modi's announcement came amid debates in countries around the world on how to lift restrictions while avoiding a spike in new infections.

French President Emmanuel Macron extended a tight lockdown in France by another month, but Italy and Austria are reopening some shops, and Spain is restarting construction and factory work.

World Health Organization chief Tedros Adhanom Ghebreyesus has warned against rushing headlong into lifting restrictions, stressing that only a vaccine can fully halt the spread.