HANOI, Vietnam (AP) — Vietnamese police arrested one of the country's wealthiest banking tycoons for undisclosed financial crimes, triggering sharp falls Tuesday on stock markets in the economically troubled nation.

The government has pledged to reform the banking sector, which experts say is laden with bad debt, poorly regulated and at possible risk of collapse.

A police officer said Nguyen Duc Kien was arrested for "wrong doings in economic activities" on Monday afternoon. The officer said he was not authorized to give his name to the media.

Kien founded Asia Commercial Bank, Vietnam's biggest private bank, but withdrew from its board in 2010.

Nguyen Thanh Toai, ACB's deputy general director, told local television that Kien was no longer a "major" shareholder in the bank and that his "arrest would not affect our banking activities."

In 2005, Standard and Chartered acquired an 8.5 percent stake in ACB.

Kien, 48, is a major shareholder in other banks, the chairman of Hanoi football club and one of Vietnam's richest men.

News of the arrest in Tuesday's papers caused a sharp drop on the country's stock markets, with the main VN-Index down by almost 5 percent at lunchtime.

Vietnam was once hailed as an emerging Asian tiger economy like Taiwan and South Korea, but concerns over the health of its banking sector are its latest black cloud. Worries over inflation, a weakening local currency, the global economic downturn and a decline in foreign investment have all led to questions over its stability.