The residents of eastern Shanghai were told to stay in their homes and face another 10 days of isolation after a four-day Covid lock down.
The new rules for Pudong were announced late on Thursday night, just hours before the lockdown was supposed to end at 5am on Friday.
It is the latest sign that China's largest city and economic powerhouse is struggling to eliminate an Omicron-driven outbreak which is the worst since the first one in early 2020.
The city said on Friday morning that it had recorded more cases on Thursday than the previous day.
The Chinese economy has been badly affected by a series of industrial centre lock-ups.
China's manufacturing activity fell to a five-month low in March as factories were forced to suspend production due to restrictions.
The BMW plants in the north-eastern city of Shenyang have been closed for more than a week because of the swine flu.
The price of oil has fallen as markets bet that the world's second economy will not need as much fuel.
The Wall Street Journal reported on Friday that there had been deaths at a care facility in the city, which Bill Bishop said had heightened concern about the outbreak in Shanghai.
He wrote in his weekly newsletter that the outbreak in Shanghai is getting worse and that it looks like the lockdowns will be extended.
If the officials are also covering up deaths then this could get really bad.
Anyone living in a building where a positive case has been found will be required to stay home for 10 more days. The residents of other buildings will be confined for three more days.
People living in a positive case area will face less restrictions. They may only be allowed to shop for necessities on certain days.
The western half of the city was due to be locked down on Friday morning as China tries to eliminate the virus completely.
16 million people will be tested in Puxi on the west side of the river. During the four-day period, residents are not allowed to leave their neighbourhoods or housing compounds, with groceries or meals delivered to their complexes.
The city's response was acknowledged by a senior official. After panic buying stripped store shelves of necessities, authorities rushed to bolster food deliveries to the city.
Ma said that they didn't prepare enough and that they were making efforts to improve it.
There was some good news in China when authorities decided to lift the citywide lock down in the province that has been hardest hit.
According to a city notice, residents of Jilin will be able to move about freely on Friday for the first time in more than three weeks. They will have to wear masks and stay 1 meter apart indoors. There are no public gatherings in parks and squares. The spread of Covid-19 has been brought under control in Jilin but not in the rest of Jilin province. Changchun, the provincial capital and an auto manufacturing hub have been locked down since March 11.
Most of the cases in the current outbreak have been in Jilin province, which borders North Korea.
There were 8,559 new cases in China in the previous 24 hours. The proportion of cases that are not related to the outbreak has been higher. Some of the new cases were imported from abroad.
With Associated Press.