The Kospi composite index tumbled to 2,450.76 points as trading opened while shares in the country's biggest company Samsung Electronics slid 3 percent before paring losses about half an hour later to 1.12 percent down.

The Korean won lost 2.5 percent against the greenback overnight to 1,444.09 against the dollar, its weakest level in two years.

It later regained ground at the start of Asian trading, ahead of an extraordinary meeting called by the South Korean central bank to "discuss the situation related to the declaration of martial law and market stabilisation measures."

At 0030 GMT, it was trading at 1,414.49 won against the dollar.

Tokyo's stocks started the day cautiously, with blue-chip Nikkei down 0.11 percent to 39,205.45 points at the open.