According to Human Rights Watch, scores of demonstrators who participated in significant anti-government demonstrations last year are still detained in China, and several are still missing.
In recent weeks, activists and media outlets have claimed that Chinese officials have surreptitiously detained an unknown number of protesters, including journalists and university students.
Demonstrators gathered in cities around the country in November to demand an end to China's severe zero-COVID laws, as well as more political freedom in some situations.
The next month, the Communist Party in power scrapped its virus containment plan, resulting in an increase in hospital admissions and deaths.
Human Rights Watch called on Beijing on Thursday to "immediately release and drop all charges against everyone detained for participating in the 'white paper' protests" referring to 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," Yaqiu Wang, senior China researcher at the U.S.-based NGO said.
"Governments and international institutions around the world should show support and call on the Chinese authorities to release them immediately."
The protests, some of which took place in big cities like Beijing and Shanghai, have received little attention from the Chinese government and official media machinery, and the detentions have not been specifically highlighted.
However, as a possible warning to protesters, a domestic law enforcement monitoring agency declared in November that it will "crackdown on illegal criminal acts that disrupt social order"
Following the unrest, security agents appeared to act quickly; friends and relatives of the participants confirmed to AFP that several arrests later occurred.
An LGBT activist's mother said that her child was detained for 30 days before being released on bail.
Human Rights Watch stated that the whereabouts and legal status of several more demonstrators are unknown.
Cao Zhixin, a 26-year-old editor at a publishing house, was detained by police after attending a vigil for the victims of a devastating fire in Xinjiang province, which sparked unrest.
Cao stated in a pre-recorded video posted to social media following her detention 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 advocacy group Chinese Human Rights Defenders said that the protests "probably indicate the tip of the iceberg" last week, stating that the amount of known detentions "are at high risk of enforced disappearance and torture."