Shanghai reported nearly 25,000 locally transmitted Covid-19 infections on Sunday, as residents of China’s most populous city voiced complaints over food and basic supplies and concern spread that more cities may soon be in the same situation.

Streets of the locked-down financial hub of 26 million people remained as curbs under the city’s “zero tolerance” policy allow only healthcare workers, volunteers, delivery personnel or those with special permission to go out.

Shanghai’s case numbers are small compared to some cities globally, but it is battling China’s worst Covid outbreak since the virus emerged in the central city of Wuhan in 2019. Of the local cases Shanghai reported on Sunday, 1,006 were symptomatic while 23,937 were classed as asymptomatic, which China counts separately.

The city has become a test bed for China’s elimination strategy, which seeks to test, trace and centrally quarantine all Covid-positive people to stem the spread of the coronavirus.

The curbs have sharply squeezed supplies of food and other essentials. Many supermarkets have been shut and thousands of couriers locked in. Access to medical care has also been a concern.

Online videos show residents struggling with security personnel and hazmat-suited medical staff at some compounds in recent days, with occupants shouting that they need food.

Executives for e-commerce giants JD.com and food delivery service Ele.me attended the city’s daily briefing, seeking to convince residents that bottlenecks would ease.

JD.com vice president Wang Wenbo said he understands concerns about delivery speed and that the company is focussing on basic foodstuffs and baby care items. Ele.me senior vice president Xiao Shuixian said his company had brought 2,800 more delivery workers in over the past week.

Citizens in several cities expressed anxiety in social media groups that their cities might also go into lockdown, with screenshots shared of maps showing various highways closed across the country.

Many of the closures may be due to local governments implementing their own measures.

On Saturday the Ministry of Transport said it met with other government departments to work on standardising highway pandemic checkpoints as restrictions at the local level were causing congestion for critical supplies, the ministry said.

A video circulating on social media appeared to show lorries departing Shanghai being scanned with-hand held detectors to make sure no one was trying to leave the city hidden inside. Reuters was not able to confirm the video’s authenticity.

Beijing’s municipal government placed a high-risk area under lockdown on Saturday after eight Covid cases were confirmed in the last two weeks, Pang Xinghuo, deputy director of the Beijing Center for Disease Prevention and Control, told reporters.

On Saturday, the southern megacity of Guangzhou, home to more than 18 million people, said it would begin testing across its 11 districts after cases were reported on Friday.

On Sunday Ningbo, a key port city near Shanghai, said it was closing all indoor dining at restaurants and hotels, and that people who had been in confined spaces would undergo testing daily for three days, without elaborating.

Guides from Shanghai circulated on social media recommended items for people in other cities to stock up on in case of lockdown, such as basic cooking equipment, seasonings, and staple dried foods like rice and pasta.