Scores of people demonstrated near HSBC's headquarters on Friday lunchtime over the authorities move to freeze millions of dollars donated by the public to support anti-government protesters.
The police arrested four people from Spark Alliance HK on Thursday, saying they were suspected of money laundering.
Last month, HSBC suspended the corporate account the group was using to collect the donations, saying the activities involved did not match the client's stated business purposes.
Protesters gathered in Statue Square opposite the HSBC Building, holding up signs calling for proof that the frozen HK$70 million were illicit funds.
A finance worker surnamed Chan said the police investigation was clearly an attempt to clamp down on the city's protest movement.
"There are so many cases, like CY Leung receiving HK$50 million ... and the DAB collecting more than HK$70 million in one night. The police have no concerns about those cases. For sure, they want to cut the financial support to the protesters," Chan said.
A financial researcher, also surnamed Chan, said the authorities are trying to make the public too scared to show their support for frontline protesters, by even stopping them from making financial contributions for those in trouble.
"They are trying to oppress the protests and oppress the civilians in different ways. They try it in a physical way and also they want to try to do it via the banking system. They are trying to put the most pressure they can on the freedom of the people," he said.
The crowd shouted slogans, sang the protest anthem "Glory to Hong Kong", and called for the government to meet the movement's five demands, including an independent inquiry into police brutality.
A number of riot officers were seen outside the bank, on the opposite side of the road to the demonstrators.