But in sharp contrast to the official Marxist line, this new generation of Marxists emphasises individual freedoms, with some even showing interest in a Western constitutional democracy – a stand the country’s mainstream Marxists and Maoists usually dismiss as the wrong path for China.

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Most of the protesters detained in August have since been released, but four have been placed under “residential surveillance at a designated location” – a form of secret detention – while four others are still in custody and could face prosecution, according to their friends and other activists.

But the whereabouts of Yue, as well as her mother, who has been out of contact since early September, remain unknown.

“Yue might not reappear for a long time,” said a student activist speaking on condition of anonymity, adding that she wanted to raise awareness of Yue’s plight.

Earlier that month, Yue had told the South China Morning Post she wanted to support the workers in Shenzhen, even if it meant being arrested.

“More than 30 innocent workers have been locked up already and treated inhumanely. I cannot just sit back and be OK with just voicing my support online – I have to go to the front line,” Yue said. “I’m prepared to be arrested … but it’s not about being arrested or not … it’s about believing that what you are doing is about justice, then you will have no fear.”

Chinese Maoists join students in fight for workers’ rights at Jasic Technology

There are also concerns for Yue’s mother. NGOCN, a social development non-profit based in Guangzhou which Yue had applied to join after graduation, said Yue’s mother had contacted them a week after the Huizhou raid, trying to find out what had happened to her daughter.

Executive director Wu Lilan said they told Yue’s mother they had not been in touch with Yue since June, but they would try to help.

“Yue Xin kept in contact with her mother after she came to Guangdong, but her mother was anxious when she hadn’t heard from her for four or five days,” Wu said.

But Wu said the NGO had been unable to contact Yue’s mother, who lives in Beijing, since September 2. The Post has also tried to contact Yue’s mother but her phone remains switched off.

An officer at Yanziling police station in Shenzhen said they were not handling Yue’s case and directed inquiries to the Pingshan district government office, which could not be reached for comment.

Apart from Yue, four editors of Epoch Pioneer, a leftist website focusing on labour activism, have been placed under “residential surveillance at a designated location” in Guangzhou, according to activists from the Jasic worker support group. They said four workers were also being held in the Shenzhen Second Detention Centre, accused of “gathering a crowd to disturb social order”, with limited access to lawyers.

Other young activists from the group have been released, but are being closely watched by their universities and parents, according to the activists.

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While classes on Marxism are routinely part of the curriculum at universities in China, the growing number of young leftist activists on campuses has apparently got college authorities worried.

Since the new academic year began in September, young Marxists have come up against tighter controls at their universities. One of them, a student at the School of Economics at Renmin University of China in Beijing, detailed how he was “blacklisted for caring about the grass roots” in an article posted online that was later removed by censors. Xiang Junwei accused faculty administrators had put him and his family under pressure, with a lecturer telling his parents he needed to correct his “dangerous thoughts” and was in a “politically problematic” position. He said he had been excluded from a university online chat group and claimed the faculty had targeted 12 other young Marxists for similar treatment.

In a separate article, he outlined the case ofanother female student at the school, who he said had been treated in hospital after she went on hunger strike to protest against her family and a lecturer trying to stop her from getting involved in the labour rights campaign in Shenzhen. He called on the university to allow her to return to her studies without having to sign a statement saying she would abandon her activism – a statement he said it insisted she sign if she wanted to go back to class.

Renmin University’s School of Economics could not be reached for comment.

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Marxist student societies have also said they had trouble renewing their registrations at top colleges including Renmin University, Nanjing University and the University of Science and Technology Beijing. At Peking University, where Yue studied, the Marxism society had to find a faculty adviser from the School of Marxism to support its application to re-register after the campus Communist Youth League chapter withdrew its backing.

Speaking in early August, Yue said students were following a tradition at the university of getting involved in grass-roots activism and labour rights. “Student activists have been fighting on a wide range of issues – including against sexual harassment and in support of democracy on campus … not everyone in this movement would identify as Marxist, Leninist or Maoist but they are certainly all influenced by Marxism,” she said.