India's 1.3 billion people will go under "total lockdown" for 21 days to combat the spread of the coronavirus pandemic, Prime Minister Narendra Modi said on Tuesday, sparking panic buying with shoppers clearing out shelves.The lockdown will begin midnight on Tuesday. India has lagged behind other nations in the number of COVID-19 cases, but there has been a sharp increase in recent days to 519 infections, including 10 deaths, according to the government.A raft of lockdown measures had already been brought in by individual states and territories - including sealing borders and restricting movement to only essential services - to try and prevent an explosion of cases that could overwhelm the fragile public health system.Modi warned that Indians had to observe the lockdown if they wanted to stop the spread of the deadly virus."This is a curfew... We will have to pay economic cost of this, but to save every family member, this is the responsibility of everyone -- the biggest priority," the prime minister added."If these 21 days are not managed, the country and your family will go back 21 years... I am not saying this as your prime minister, I am saying it as your fellow citizen, family member."Under the order, people could face up to two years in jail and an unspecified financial penalty if they flout the rules.'Don't panic' Within minutes of Modi's speech announcing the world's largest lockdown, AFP reporters witnessed people rushing to stores to buy groceries.In India's capital New Delhi, some people in their pyjamas ran towards nearby shops despite heavy rain to stock up, while in the financial capital Mumbai, long lines of masked people formed outside pharmacies and supermarkets.In Ghaziabad, a city in India's most populous state Uttar Pradesh, police patrolled streets and used loud megaphones to tell residents to stay indoors.Following his speech, Modi tweeted: "THERE IS ABSOLUTELY NO NEED TO PANIC."My fellow citizens, THERE IS ABSOLUTELY NO NEED TO PANIC.Essential commodities, medicines etc. would be available. Centre and various state governments will work in close coordination to ensure this.Together, we will fight COVID-19 and create a healthier India. Jai Hind!— Narendra Modi (@narendramodi) March 24, 2020 "Essential commodities, medicines etc. would be available. Centre and various state governments will work in close coordination to ensure this," he added.Local media reported that state leaders would announce the timings of when people could buy essential items.