It's the first time Shanghai has imposed such strict restrictions - until the latest move, it had taken a more relaxed approach than other Chinese cities.
People in the city are confined to their homes, and most have to order in food and water and wait for government deliveries of vegetables, meat and eggs.
Videos shared on social media show complaints by angry residents about food shortages and inadequate medical supplies.
It's Shanghai's first experience of a city-wide lockdown. It previously tackled growing infection rates through smaller localised lockdowns.
This typically meant individual residential complexes - each housing several hundred people - were locked down instead of the entire city. And for a while, it looked like this method was working.
Even when case numbers rose to nearly 1,800 in March 2021, Shanghai did not impose a full lockdown.
By comparison, Xi'an, which is home to nearly 13 million people, sealed the entire city after less than 100 cases in December 2021.
The city of Yuzhou, in Henan province, locked down more than 1.1 million people after just three confirmed Covid cases.
But in late March, Shanghai began seeing huge numbers - with more than 2,500 cases recorded on 27 March alone.
The city decided to announce a phased lockdown, firstly in its eastern district, and then its western district days later.
Shanghai has now relaxed some restrictions by dividing the city into three zones - lockdown zone, controlled zone and precautionary zone.
Areas which have no new cases in a week will be marked as "control", and if they reach two weeks with no new cases, they will be further downgraded to "prevention".
People in the zones with fewer or no cases will have more freedom, such as being allowed to move around within their housing compounds.
On 19 April, Shanghai Health Commission announced that two districts had achieved "Zero-Covid case" and that four million people were living in control and prevention zones.
Additional reporting by Tim Bowler; Graphics by Rob England and Jana Tauschinski
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