Hong Kong police have frozen nearly $10 million belonging to a group raising funds on behalf of pro-democracy protesters.

Police also arrested four members of the group, Spark Alliance, on money laundering charges, according to reports.

Spark Alliance accused the police of “smears” in a Facebook post, the Guardian reported. The freeze and arrests were announced Thursday.

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The paper quoted police as alleging that some of the donations were used by the fund's owners for other investments.

“We found the donated money was transferred to a shell company and a significant portion of this money was invested in personal insurance products,” Police Superintendent Chan Wai-kei said, according to the paper. “The beneficiary of these products is the person in charge of the shell company.”

Hundreds of anti-government activists gathered outside a Hong Kong detention center Friday demanding the release of the arrested Spark Alliance members and denouncing the funds' freeze, Reuters reported.

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Spark Alliance was launched in 2016 to support protesters on a number of fronts including legal fees, according to the news outlet.

Chan said suspicions were raised after officers monitored large cash deposits being made on behalf of a shell company, as well as the purchasing of insurance policies. He said the deposits were “incommensurate with (the company's) business nature," while the money used to buy the insurance far exceeded the purchaser's personal income.

“Based on these money laundering hallmarks, we made arrests today,” Chan said.

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More than 6,100 people have been arrested in relation to the mass demonstrations that began in June in response to proposed legislation that would have allowed Hong Kong residents to be tried in mainland China.

The Associated Press contributed to this report.