* Beijing kicks off first of three rounds of mass testing in populous district
* Mass testing fuels concerns of shortages of food, supplies
* Chinese stock market down on fears Beijing may join Shanghai in lockdown (Adds official comments on replenishment of supermarket goods)
BEIJING, April 25 (Reuters) - Beijing residents snapped up food and other supplies as the city's biggest district began mass COVID-19 testing of all residents on Monday, prompting fears of a Shanghai-style lockdown after dozens of cases in the capital in recent days.
Authorities in Chaoyang, home to 3.45 million people, late on Sunday ordered residents and those who work there to be tested three times this week as Beijing warned the virus had "stealthily" spread in the city for about a week before being detected.
"I'm preparing for the worst," said a graduate student in the nearby Haidian district surnamed Zhang, who placed online orders for dozens of snacks and 10 pounds of apples.
Shoppers in the city crowded stores and online platforms to stock up on leafy vegetables, fresh meat, instant noodles and rolls of toilet paper.
In Shanghai, where most of its 25 million residents have been locked down for weeks, the main food supply bottleneck has been the lack of enough couriers to make deliveries to homes, fuelling anger among residents.
In Beijing, supermarket chains including Carrefour and Wumart said they had more than doubled inventories, while Meituan's grocery-focused e-commerce platform increased stocks and the number of staffers for sorting and delivery, according to the state-backed Beijing Daily.
Supermarket chains should ensure that goods were being replenished in time, said a Beijing official at a news conference late in the day, adding that the city's reserves of refined grains and oil could meet the consumption needs of residents for 30 days.
The operating hours of stores would also be extended, the official said.
Since Friday, Beijing has reported 70 locally transmitted cases in eight of its 16 districts, with Chaoyang accounting for 46 of the total, said a local health official on Monday.
Even in districts such as Haidian that have yet to report any cases in the current outbreak, there is a sense of growing unease over food supply.
AREAS UNDER LOCKDOWN
While the Chinese capital's caseload is small compared with those globally and the hundreds of thousands in Shanghai, Chaoyang district told residents to reduce public activities, although most schools, stores and offices remained open.
Chinese shares tumbled on Monday, with the blue-chip CSI300 index closing down 4.9% at a two-year low, weighed by worries Beijing was on the verge of joining Shanghai in lockdowns.