Chinese authorities are using cellphone data to track down protesters who demonstrated against the government’s stringent covid restrictions in Beijing, according to a recording of a phone call between a protester and police heard by CNN.

Hundreds of demonstrators gathered along Beijing’s Liangma River on Sunday night, calling for an end to the incessant covid tests and lockdowns. Some also decried censorship and called for greater political freedoms. The extraordinary show of dissent in the heart of the Chinese capital ended largely peacefully in the early hours of Monday.

But some protesters have since received phone calls from the police inquiring about their participation. A protester told CNN they received a phone call Wednesday from a police officer, who revealed they were tracked because their cellphone signal was recorded in the vicinity of the protest site. CNN is not naming any of the protesters in this story to protect them from reprisals.

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