Authorities in Chaoyang, home to 3.45 million people, late on Sunday ordered residents and those who work there to undergo testing this week as Beijing warned the virus had "stealthily" spread in the city for about a week before being detected.
Since Friday, Beijing has reported 47 locally transmitted cases, with Chaoyang accounting for more than half of them.
While the Chinese capital's caseload is small compared to those globally and the hundreds of thousands in Shanghai, Chaoyang district told residents to reduce public activities and suspended in-person private tutoring classes.
However, most schools, stores and offices remained open.
The Chaoyang district is home to many wealthy residents, most foreign embassies as well as entertainment venues and corporate headquarters. It has little manufacturing.
"The current outbreak in Beijing is spreading stealthily from sources that remained unknown yet and is developing rapidly," a municipality official said on Sunday.
More than a dozen buildings in Chaoyang have been put under lockdown. For the rest of the district, people will be tested on Monday and again on Wednesday and Friday.
People queued up at makeshift sites manned by medical workers in protective suits as testing began.
"I came as the notice suggested, at 6 a.m., for testing just to make sure that I can get to work on time," said a man in his 30s queuing for a test in his residential compound.
Supermarket chains including Carrefour and Wumart said they had more-than-doubled inventories, and extended opening hours on Sunday, while Meituan's grocery-focused e-commerce platform increased inventory and the number of staffers for sorting and delivery, according to the state-backed Beijing Daily.
In Shanghai, where most of its 25 million residents have been locked-down for weeks, the main food supply bottleneck has been a lack of enough couriers to make deliveries to homes.
A graduate student surnamed Zhang in Beijing's Haidian district on Sunday placed online orders for dozens of packets of snacks and 10 pounds of apples, concerned about China's tough policy against the virus and Omicron's transmissibility even though the current focus is on Chaoyang.
"I'm preparing for the worst," he said.