Beijing has recorded 36 new locally-transmitted coronavirus cases, amid fears of a second wave in the Chinese capital.
Another 36 cases were also recorded on Saturday. The city had previously seen no new cases in more than 50 days.
The country's Vice Premier Sun Chunlan called on officials to take "decisive measures", warning that the risk of further spread remained high.
The outbreak has been linked to the city's largest wholesale market.
The general manager of the market has been dismissed, along with other local officials.
Three other provinces - Liaoning, Hebei and Sichuan - have also reported confirmed or suspected cases connected to Beijing.
Local media reports say the virus was discovered on chopping boards used for imported salmon at the market, prompting major supermarkets in Beijing to pull the fish from their shelves.
According to China's National Health Commission, Beijing recorded one new virus case on Thursday and six on Friday - the first cases in almost two months.
On Saturday, 36 new local cases were recorded in Beijing, all related to Xinfadi market - which has been described by state media outlet CGTN as the biggest wholesale market in all of Asia.
The market was quickly placed under lockdown and restrictions were imposed in 11 nearby neighbourhoods.
On Monday, ten more neighbourhoods around the market were restricted, said CGTN. No visitors or deliveries are allowed, but residents can come and go.
Schools and nurseries near the market were told to shut and the re-opening of primary schools, originally scheduled for today, has now been postponed, reported the Global Times.
The best and worst of the Chinese Communist Party
Analysis by Stephen McDonell, China correspondent, Beijing
The number of infections may not sound huge at the moment.
But, after more than 50 days with no cases at all, the authorities here are worried this could easily turn into a full-blown coronavirus second wave in the city.
This is especially the case given these infections are being traced back to a massive wholesale market which has tens of thousands of visitors every day, supplying 80% of Beijing's meat and vegetables.
In the response, we are again seeing the best and worst of China's Communist Party under Xi Jinping.
The speed and scale of isolation measures and mass testing has been impressive. However, the dismissals - not so much.
The deputy head of Fengtai district government has been removed, as has the local party secretary for Huaxiang area.
The general manager of Xinfadi market has been dismissed and other officials from the market summoned by the party's discipline inspection commission.
Here's a question: if you really wanted to encourage a culture with cover-up as your default setting, where fear trumped openness, how might you achieve that?
One way would be to sack any leader with a cluster of coronavirus cases suddenly appeared - because the arrival of this invisible killer must be their fault.
The chief epidemiologist of China's Centre for Disease Control and Prevention (CDC) said the virus strain found in Beijing did not resemble the type circulating across the rest of the country, suggesting it might have been brought in from elsewhere.
Some 10,000 market staff will be tested for the virus.
This potential new wave of cases came as normal life had begun resuming across Beijing and most parts of China.
People had cautiously returned to workplaces and students back to schools - though virus restrictions still remained in place.