At least a dozen young Chinese activists who participated in a national workers’ rights campaign are missing, their friends said on Sunday, in an apparent effort by China’s government to silence one of the most prominent student protests in years, The New York Times reported on Sunday.

Unidentified men in at least five Chinese cities rounded up the activists, who are recent graduates of elite universities, over the past few days, according to their friends. The men assaulted several activists before shoving them into vehicles and driving away, the friends said.

The activists, who describe themselves as loyal communists who fervently believe in the ideals of Marx and Mao, have waged an unorthodox campaign against inequality and corporate greed that has attracted a following at some of China’s best universities.

The campaign has put the ruling Communist Party, which prides itself as a socialist guardian of workers’ rights, in an uncomfortable position. In line with President Xi Jinping’s efforts to curb dissent and political organizing, the party appears to be stepping up is efforts to crush the movement.

Patrick Poon, a researcher at Amnesty International in Hong Kong, said, “We call on the authorities to immediately release the students and supporters, and allow an independent investigation of what is happening to them.”

It was unclear what happened to the activists, who were rounded up in Beijing, Guangzhou, Shanghai, Shenzhen and Wuhan.