Millions of Indians turned off their lights and lit up balconies and doorsteps with lamps, candles and flashlights on Sunday, in response to Prime Minister Narendra Modi's appeal to "challenge the darkness" spread by the coronavirus crisis.

Modi, who imposed a three-week nationwide lockdown from March 25, asked all citizens to turn out their lights for nine minutes at 9pm local time (15:30 GMT) on Sunday, and to display lamps and candles in a show of solidarity.

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Modi's call was met with a huge response, with many people lighting up their balconies. Others lit firecrackers, played musical instruments, and sang patriotic songs. Some residents turned on the torch on their mobile phones.

Residents shouted "Hail mother India" and "Go corona go". They cheered, clapped, and set off firecrackers and fireworks that shattered the quiet evening, in scenes reminiscent of Diwali, the Hindu festival of lights.

"Salute to the light of the lamp which brings auspiciousness, health and prosperity, which destroys negative feelings," Modi tweeted in Sanskrit. He shared photos of himself lighting a diya (clay lamp).

A family in Ahmedabad holds lamps to mark the country's fight against the pandemic [Ajit Solanki/The Associated Press]

Modi had on Friday called on Indians to take part in the event to "defeat the deep darkness of the crisis by spreading the glory of light in all four directions".

Grid data showed India's national power consumption plunging more than a quarter in a matter of minutes.

Lockdown measures

The show of unity came as the total number of coronavirus cases in India crossed 4,000 on Sunday, while the death toll rose to 109, according to Indian media reports.

Experts say wider testing for coronavirus disease is needed.

India has been under a strict lockdown since March 25, halting public transport and closing offices and shops apart from essential services.

But the shutdown has badly hit rural migrant workers, leaving them jobless with hundreds of thousands trying to return to their villages, many on foot.

Many others have been blocked from returning home. The government has set up tens of thousands of makeshift camps and feeding centres for them.

Some officials have warned the lockdown could continue beyond April 14 in parts of India where new cases have been detected.

The health ministry has blamed a recent jump in cases on a large religious gathering in the capital New Delhi in March that has been linked to more than 1,000 infections.

"If the Nizamuddin incident [in Delhi] had not happened, then our case doubling rate which is currently 4.1 would have been 7.4 days," health ministry official Lav Agarwal told reporters on Sunday.

The ministry on Saturday issued an advisory for Indians to use homemade reusable face covers to "help in protecting the community at large".

The government imposed a ban on the export of malaria treatment drug hydroxychloroquine, removing exemptions such as for fulfilling export obligations that it put in place several days ago.

Hydroxychloroquine and chloroquine - used to treat malaria - have shown early promise against the COVID-19 illness in early studies in France and China.