The United States has urged Chinese authorities to release the three men, who were investigating two plants owned by footwear producer Huajian Group when they were detained last month.
China confirmed this week that they were being investigated on suspicion of using "spying and other monitoring equipment".
A Chinese media report on Wednesday cited police as saying the men, who worked on behalf of New York City-based China Labor Watch, had confessed to using watch-like recording devices to collect "business secrets" at the factories.
But Amnesty International said the men were likely taken into custody to prevent the publication of potentially "embarrassing information" about a supplier connected to US President Donald Trump's daughter.
Relations between China and the United States have shown signs of improvement in recent months as Trump softens the anti-Beijing rhetoric he used on the campaign trail.
Two of the activists -- Hua Haifeng and Su Heng -- were being held in a detention centre in the southern city of Guangzhou, said Nicholas Bequelin, Amnesty's regional director for East Asia.
The group was confident that the third -- Li Zhao -- was also being detained at the same place.
"We are urging China to release them immediately and otherwise afford them judicial and fair trial protections to which they are entitled," US State Department spokeswoman Heather Nauert said.
But China has brushed off Washington's demands, with foreign ministry spokeswoman Hua Chunying telling reporters: "No country can interfere in China's sovereign and judicial independence."
The two factories under investigation by the activists were in the city of Dongguan in the southern province of Guangdong, and Ganzhou in neighbouring Jiangxi province.
They discovered that employees were forced to work overtime while being paid less than the minimum wage, according to China Labor Watch director Li Qiang, who has alleged that the factory issued fake pay slips that showed wages greater than the workers' actual compensation.
Aside from Ivanka Trump, Huajian also manufactures products for Coach, Nine West, Karl Lagerfeld, and Kendall + Kylie, China Labor Watch's Li said.
A spokesman for the company told AFP in October that it had made about 100,000 pairs of Ivanka Trump-branded shoes over the years.