Gold trading is unregulated in Malaysia and the central bank says more than 20 so-called gold investment companies are involved in making unauthorised transactions.
Although they have been blacklisted, tens of thousands of people have already sunk their money into these companies.
The bank says it suspects any money they receive in return is not made from gold trading, but from other investors' payments.
However, the bank's decision to seize four of the companies' assets has angered investors.
They are planning a rally later this week demanding the return of their gold and their money.
Al Jazeera's Florence Looi reports from Kuala Lumpur.