Here's your 10-point cheatsheet to this big story:
Normally bustling streets in New Delhi and Mumbai were mostly deserted as the shutdown began at 7:00 am. The one-day "janata curfew", one of the biggest national exercises in the world, has been necessitated by the rising cases of COVID-19 across India. On Thursday, PM Modi had said in a 29-minute address to the nation: "There is no cure for this, so we need to remain healthy. We need to avoid crowds and stay home. Social distancing is critical. If you think you can roam around as usual and feel you will not be at risk, this is incorrect; you are endangering yourself and your family."
The PM asked citizens to stand at their doors, windows and balconies at 5 pm and clap, clang utensils or ring bells in a show of support to those providing essential services while the entire country is on a self-quarantine mode.
A woman in Pune and a man in West Bengal - both without any history of foreign travel - tested positive for coronavirus on Saturday, raising worries about possible community transmission as COVID-19 cases in India climbed to 315 on Saturday.
Except businesses and government establishments dealing in essential items and services, all others will remain shut today. Businesses have voluntarily announced they would follow the PM's call for self-quarantine.
Transport will be affected across the country. No trains - long-distance or suburban - will run, the railways has said, but those already running won't be stopped during the day. Several airlines including IndiGo and GoAir have said they will either operate in minimum capacity or cancel all flights. No metro trains will run in Delhi, Bengaluru and other cities.
The gradual shutdown across the country - Rajasthan is the latest in the long list of states that have announced safety curbs - has hit businesses hard. The travel sector has taken the biggest hit with booking cancellations.
The railways has asked people to postpone all journeys after 12 people who travelled this month were found to be infected with coronavirus. Lakhs of people use the railways everyday; the coaches are often full of people, which raises the risk of the coronavirus spreading very fast.
There has been much criticism against people who have come from abroad and skipped quarantine. The teen son of an IAS Officer in Kolkata, who returned from the UK, possibly came into contact with many before testing positive for coronavirus. The family of the wife of a Google employee in Bengaluru, whose husband tested positive for coronavirus, allegedly misled health authorities over her whereabouts. A police complaint has been filed against singer Kanika Kapur, who attended parties and has caused panic among several politicians and others who were at a dinner with her.
On Saturday, people posted videos on social media of their dry run of the 5 pm clapping and clanging from balconies to show appreciation to health professionals and other workers who are at the frontline in the war against COVID-19.
Coronaviruses are a large family of viruses that cause illness ranging from the common cold to more severe diseases. The COVID-19 is a new strain that was discovered last year in China's Wuhan and has not been previously identified in humans. Coronaviruses are zoonotic, meaning they are transmitted between animals and people.