Photos and videos circulating on social media showed hundreds of protesters in central Beijing on Monday, chanting slogans and holding red banners urging President Xi Jinping to stop the investigation into the group.
While the authenticity of the images could not be verified, Beijing police essentially confirmed that a protest was held in a statement saying that scores were detained following the "illegal gathering".
Sixty-three members of the pyramid scheme, known as Shanxinhui, were held for "harming social management order" and four more for "disrupting order in a public place," the Beijing police said in a statement on their official Weibo microblog.
Protests of such size are rare in Beijing, where authorities have little tolerance for public demonstrations.
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On Friday, police had arrested Zhang Tianming, the group's founder, and several employees on charges of defrauding "a huge amount of property" from victims "under the guise of 'helping the poor and achieving common wealth,'" according to a police statement.
But banners carried by protesters at the protest claimed Shanxinhui was a legitimate platform that was being "persecuted".
A cached version of the website described the organisation as an equity investment group founded in 2013 that promotes causes such as forestry conservation and poverty alleviation.
Pyramid schemes have become more popular in China in recent years, with promotional material spreading rapidly on social media networks.
Police investigated 2,826 pyramid scheme cases in 2016, nearly 20 percent more than in 2015, according to the Ministry of Public Security.
From 2005 to 2015, 21,904 pyramid schemes were investigated and 990 million yuan (USD 140 million) was confiscated, the ministry said.