Videos on social media appear to show some protesters asking leader Xi Jinping to resign.
Protests against Covid restrictions in China appear to have intensified following a fire which killed 10 people in an apartment block in Urumqi.
In the city of Shanghai, videos posted on social media by foreign journalists show thousands of people taking to the streets to remember the victims and to protest against Covid restrictions.
Hundreds were heard asking for President Xi Jinping to resign.
Many people have blamed the lockdown of residential buildings for the fire.
Chinese authorities have denied it was the cause. Though Urumqi authorities did issue an unusual apology late on Friday – vowing to punish anyone who had deserted their duty.
At the gathering in Shanghai, some people were seen lighting candles and laying flowers for the victims.
Others were heard shouting slogans such as “Xi Jinping, step down” and “Communist party, step down”. Some also held blank white banners.
Such demands are an unusual sight within China, where any direct criticism of the government and the president can result in harsh penalties.
Some protesters also shouted abuse at police, who lined the streets where people had gathered.
One protester told the Associated Press news agency one of his friends had been beaten by police at the scene, while two others had been pepper sprayed. Videos of other parts of the protest showed police standing watch as people protested.
Though the situation in the area had calmed by Sunday morning, the BBC saw a heightened police presence in the area of the protest, with several dozen police officers, private security guards and plain-clothed police officers on the streets.
Elsewhere, in several Chinese universities, photos and videos emerged online of students protesting on Saturday night. The largest gathering seemed to be at Nanjing Communications University.
Videos of the protests are difficult to independently verify, but many of them show an unusually explicit and outspoken criticism of the government and its leader.
The protests are the latest in an accelerating series of mass demonstrations against China’s zero-Covid measures which have also become increasingly bolder in criticism of the government and President Xi.
The zero-Covid strategy is the last policy of its kind among the world’s major economies, and is partly due to China’s relatively low vaccination levels and an effort to protect elderly people.
In spite of the stringent measures, China’s case numbers this week hit all-time records since the pandemic began.