BEIJING — China’s capital Beijing began testing millions of residents and shutting down residential and business districts Monday amid a new outbreak of COVID-19.

While only 70 cases have been found so far in the city of more than 21 million since the outbreak surfaced Friday, authorities have implemented extreme measures to prevent a further spread of the virus.

Residents were staying home and stocking up on food as a safeguard against the possibility that they could be confined indoors, as has happened in multiple cities including the financial hub of Shanghai.

Shanghai, which has been locked down for more than two weeks, reported more than 19,000 new infections and 51 deaths in the latest 24-hour period, pushing its death toll from the ongoing outbreak to well over 100.

Long lines formed in supermarkets in central Beijing. Shoppers snapped up rice, noodles, vegetables and other food items, while store workers hastily restocked some empty shelves. State media issued reports saying supplies remained plentiful in Beijing despite the buying surge.

Shoppers appeared concerned but not yet panicked. One woman, carrying two bags of vegetables, eggs and frozen dumplings, said she is buying a little more than usual. A man said he isn’t worried but is just being cautious since he has a 2-year-old daughter.

Beijing health officials said 29 more cases had been identified in the 24 hours through 4 p.m. Monday, raising the total to 70 since Friday.

The city has ordered mass testing across sprawling Chaoyang district, where 46 of the cases have been found. The more then 3 million residents of Chaoyang, as well as people who work in the district, need to be tested on Monday, Wednesday and Friday.

Testing sites were set up overnight and in the early morning at residential complexes and office buildings around Chaoyang district.

“I think Beijing should be fine,” said Gao Haiyang as he waited on line for a COVID-19 test. “Based on previous response made by my community, if there’s any emergency, I think supply can be guaranteed. Plus there were lessons we learned from other cities. I think we can make good preparations.”

The central city of Anyang, along with Dandong on the border with North Korea, also announced lockdowns as the omicron variant spreads across the vast country.

China’s borders remain largely closed as its hardline response and the pandemic’s economic impact continue to grow.