SHENZHEN, China (Reuters) - A Chinese labor activist and his assistant who ran a social media account helping sanitation workers in the southern city of Guangzhou have been arrested, friends said on Thursday.

Chen Weixiang ran a blog called “Heart Sanitation” which provided advice to workers on how to deal with unfair contracts and gain public attention for their working conditions by using social media platforms.

Chen and his assistant were arrested and taken from Chen’s home by more than a dozen uniformed and plainclothed police on Tuesday, a fellow activist and friend, Yang Zhanqing, who is based in the United States, told Reuters.

Since then, friends have heard no news about their alleged crimes or their whereabouts.

A third researcher who helped maintain the social media account had gone missing, Yang said.

Chen’s disappearance was confirmed by another friend, who declined to be identified.

Police in Guangzhou responsible for Chen’s home district did not comment and the security bureau in the area could not immediately be contacted.

The detentions follow a wider crackdown on worker support groups in China this year, the China Labor Bulletin said, with police raids on labor groups in Beijing, Guangzhou and Shenzhen.

China’s sanitation workers face some of the country’s harshest working conditions, with low or unpaid wages and dangerous working conditions common, said William Nee, a business and human rights advisor at Amnesty International.

“Workers in this sector are often rural migrant workers with low levels of education and with scarce resources, so claiming their rights within the legal system without assistance can be difficult,” Nee said.

Chen’s assistant, who Yang identified as Miss Lu, started volunteering with Chen in the last few months.

The third assistant, identified as Mr Zhang, is a graduate student studying in Taiwan who was collecting material for his thesis. He had also been unreachable, Yang said.

(THis story corrects name of activist in paragraph 2)