India's 1.3 billion people were under an immense lockdown on Wednesday to prevent an explosion of coronavirus cases as the government struggled to deliver essential goods and calm nerves, Reuters reports.

Prime Minister Narendra Modi ordered everyone indoors for the next three weeks, saying this was the only way to avoid a disaster and effectively shutting down Asia's third-largest economy.

But with less than four hours before the decree went into effect at midnight on Tuesday, crowds of people thronged shops to stock up on milk, rations and medicines even though Modi said these would be available throughout the lockdown.

Others including labourers and day workers headed home to the countryside from cities such as Delhi, many walking because public transport also stopped.