In a statement, ACB Vice Chairman Trinh Kim Quang said Kien's arrest was "personal."
The government said Kien, 48, was arrested on suspicion of carrying out "illegal business," The Financial Times reported Tuesday.
The country's central bank reacted to the arrest, as well, issuing a statement that said, "The State Bank of Vietnam is closely monitoring developments in the money market. If necessary, the State Bank of Vietnam is willing to offer liquidity support to ensure the stability and safety of the system."
The VN-Index of the Ho Chi Minh Stock Exchange fell nearly 4 percent, while the HNX-Index of the Hanoi Stock Exchange fell nearly 5 percent after the arrest. Financial stocks fell sharply, the Times said.
Kien is one of the country's most successful businessmen. He no longer holds a position at ACB, but he is chairman of the popular Hanoi Football Club and the majority stockholder in several banks, including ACB and Kien Long Bank, Vietnam.net Bridge reported.