People in China who attended weekend protests against Covid restrictions say they have been contacted by police, as authorities begin clamping down.
Several people in Beijing said police had called demanding information about their whereabouts.
It is unclear how police might have discovered their identities.
On Tuesday officials renewed a promise to speed up efforts to vaccinate older people. Vaccination rates among elderly people are relatively low.
China has recorded record numbers of new cases in recent days.
Over the weekend, thousands in China took to the streets demanding an end to Covid lockdowns – with some even making rare calls for President Xi Jinping to stand down.
But on Monday, planned protests in Beijing did not happen after officers surrounded the assembly point. In Shanghai, large barriers were erected along the main protest route and police made several arrests.
The demonstrations began after a fire in a high-rise block in Urumqi, western China, killed 10 people on Thursday. Many Chinese believe Covid restrictions contributed to the deaths, although the authorities deny this.
Asked whether the protests would prompt a change to zero-Covid rules, an official said China would continue to “fine tune and modify” its measures.
“We are going to maintain and control the negative impact to people’s livelihoods and lives,” said Mi Feng, a National Health Commission spokesman, at a press conference.
On Tuesday morning, police could be seen in both Beijing and Shanghai patrolling areas where some groups on the Telegram messaging app had suggested people should gather again.
A small protest in the southern city of Hangzhou on Monday night was also quickly stopped with people swiftly arrested, according to social media footage verified by the BBC.
Reports also say that police were stopping people and searching their phones to check if they had virtual private networks (VPNs) set up, as well as apps such as Telegram and Twitter which are blocked in China.
One woman told news agency AFP that she and five of her friends who attended a protest in Beijing had received phone calls from police.
In one case, a police officer visited her friend’s home after they failed to answer their phone and asked whether they had visited the protest site, stressing that it was an “illegal assembly”.
Another told Reuters that they were asked to show up at a police station to deliver a written record of their activities on Sunday night.
“We are all desperately deleting our chat history,” one Beijing protester told Reuters. “Police came to check the ID of one of my friends and then took her away. A few hours later they released her.”
Police have also detained journalists covering the protests in recent days. News agency Reuters said one of its journalists was briefly detained on Sunday before being released.
BBC journalist Ed Lawrence was also held for several hours while covering a protest in Shanghai on the same night. UK Prime Minister Rishi Sunak said his detention was “shocking and unacceptable”, adding that Britain would raise concerns with China about its response to the protests.