China's financial hub of Shanghai launched a planned two-stage lockdown of the city of 26 million people on Monday (Mar 28), closing bridges and tunnels, and restricting highway traffic in a scramble to contain surging local COVID-19 cases.

The snap lockdown, announced by Shanghai's city government on Sunday, will split the city in two roughly along the Huangpu River for nine days to allow for "staggered" testing. It is the biggest COVID-19-related disruption to hit the city so far.

The Pudong financial district and nearby areas will be locked down from early on Monday to Friday as citywide mass testing gets under way, the local government said.

In the second phase of the lockdown, the vast downtown area west of the Huangpu River that divides the city will then start its own five-day lockdown on Friday.

Residents will be required to stay home, and deliveries will be left at checkpoints to ensure that there is no contact with the outside world.

The announcement marks a turnaround for the local government, which last week expressly denied that Shanghai would be locked down as it pursued a more piecemeal "slicing and gridding" approach to try to prevent infections from spreading.

Wu Fan, a member of Shanghai's expert COVID-19 team, told a briefing on Monday that recent mass testing had found "large-scale" infections throughout the city, triggering a stronger response.

"Containing the large-scale outbreak in our city is very important because once infected people are put under control, we have blocked transmission," she said, adding that testing would be carried out until all hidden risks are eliminated.

A record 3,450 asymptomatic COVID-19 cases were reported in Shanghai on Sunday, accounting for nearly 70 per cent of the nationwide total, along with 50 symptomatic cases, the city government said on Monday.

Shanghai's Public Security Bureau said that it was closing cross-river bridges and tunnels, and highway tollbooths concentrated in the city's eastern districts through Apr 1. Areas to the west of the Huangpu River will have similar restrictions imposed from Apr 1 to Apr 5.

In a statement posted to its official Weibo account, the bureau said that traffic controls would be implemented on highways into and out of the city, requiring people leaving Shanghai to show proof of negative results from nucleic acid tests taken within 48 hours.

The city government said on Sunday that it would suspend public transport, including ride-hailing services, in locked-down areas. It also ordered the suspension of work at firms and factories, with the exception of those offering public services or supplying food.

Shanghai’s Disney theme park is among the businesses that closed earlier.

American automaker Tesla is suspending production at its factory in the city for four days, two people familiar with the matter told Reuters. Tesla did not immediately respond to a request for comment on Monday.