The move to prevent the demonstration against the state-managed Fanya Metals Exchange reveals the authorities' sensitivity to unrest linked to financial losses in China, where stock markets have plummeted in recent months, wiping trillions off valuations.
The protest was planned for Monday and would have coincided with the start of a key Communist Party meeting in Beijing.
Yu Haichao was among more than 2,000 investors who travelled to Beijing to take part, but police knocked on the door of her Beijing hotel room around midnight and told her they would take her to a meeting to resolve the dispute, she said.
"I was kept in a cold room with about 100 others without food, water or heat," Yu said. "They used such a show of force, and we didn't even break the law."
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"I was terrified, I feel completely helpless," she added. "I will never protest again."
The protesters were forced by police to sign a pledge that they would not attend any gathering related to protest about the exchange, she said, and the following day officials took her back to her home province Shanxi.
While most of the Fanya protesters were sent back to their home towns, accompanied by local government officials, about a dozen have been held by police and formally detained, said Zhu.
Zhu said he had invested one million yuan ($157,000) in the exchange after seeing state television endorsing the project.