BEIJING — A labor activist who had been working undercover at a Chinese factory that makes shoes for Ivanka Trump and other brands has been detained by the police, as concerns rise over a crackdown on the country’s advocacy groups.

The activist, Hua Haifeng, who was working on behalf of the advocacy group China Labor Watch, was detained on suspicion of illegal eavesdropping, his wife, Deng Guimian, said late Tuesday. Mr. Hua and two other labor activists had been undercover at two Chinese factories that make shoes for Ms. Trump and other brands. They all disappeared on Saturday, according to their employer, China Labor Watch.

They were last seen in Ganzhou, a large city in southern Jiangxi Province. It is unclear whether the other two were detained.

The group had been investigating labor conditions at the two factories, one in Ganzhou and another in Dongguan. Their preliminary findings, which had not yet been made public, detail that workers were subject to exceptionally long hours.