Dozens of demonstrators are still detained in China after taking part in widespread protests against the government last year, Human Rights Watch said Thursday, adding that the whereabouts of some remain unknown.
Protesters gathered in cities across the country in November to call for an end to China's hardline zero-Covid restrictions, in some cases also demanding greater political freedoms.
The ruling Communist Party abolished its virus containment strategy the following month, after which a wave of infections caused a spike in hospitalisations and deaths.
Campaigners and media outlets have reported in recent weeks that Chinese authorities have quietly detained an unknown number of protesters, including university students and journalists.
On Thursday, Human Rights Watch urged Beijing to "immediately release and drop all charges against everyone detained for participating in the 'white paper' protests", referencing the blank sheets held in defiance of state censorship.
"Young people in China are paying a heavy price for daring to speak out for freedom and human rights," said Yaqiu Wang, senior China researcher at the US-based NGO.
"Governments and international institutions around the world should show support and call on the Chinese authorities to release them immediately."
- Swift crackdown -
China's government and state media apparatus have made scant acknowledgement of the protests -- some of which occurred in major cities such as Beijing and Shanghai -- and have not directly mentioned the detentions.
But an oversight body for domestic law enforcement said in November that it would "crack down on illegal criminal acts that disrupt social order" in a possible warning to demonstrators.
Security services appeared to move swiftly in the wake of the unrest, with participants' friends and relatives confirming to AFP that several arrests subsequently took place.
One mother said her child -- an LGBT activist -- was held for 30 days before being released on bail.
The whereabouts and legal status of several other demonstrators remain a mystery, Human Rights Watch said.
They include 26-year-old Cao Zhixin, an editor at a publishing house who was taken into police custody after attending a vigil for the victims of a deadly fire in Xinjiang province, which became a trigger for unrest.
In a pre-recorded video uploaded to social media after her detention, Cao said that several of her friends had also been detained and could not be contacted.
"Don't let us vanish from this world. Don't let us get taken away or convicted of a crime arbitrarily," she said.
The campaign group Chinese Human Rights Defenders said last week that the number of known detentions "probably indicate the tip of the iceberg", warning that the demonstrators "are at high risk of enforced disappearance and torture".