The KOSPI fell more than 3 percent on net foreign selling of about 3.6 trillion won and a sharp drop in heavyweight semiconductor stocks, sliding below the 6,700 level. The Kosdaq, by contrast, rose more than 1 percent, diverging from the main board.

According to the Korea Exchange on Monday, the KOSPI ended down 215.99 points, or 3.12 percent, at 6,696.96. The index opened down 31.88 points, or 0.46 percent, at 6,881.07 and gradually extended its losses.

In the main board market, foreign investors drove the decline, net selling 3.7 trillion won. Institutions also sold 1.3 trillion won. Retail investors net bought 3.3 trillion won, absorbing supply from foreigners and institutions.

Among the top market-cap stocks, weakness in semiconductor shares stood out.

Samsung Electronics closed down 8.70 percent at 257,000 won. SK Hynix ended down 3.41 percent at 1,671,000 won.

SK Square fell 4.19 percent to 1,076,000 won, and Samsung C&T dropped 7.84 percent to 364,500 won. Hyundai Motor and KB Financial Group also fell 0.24 percent and 0.73 percent, respectively.

LG Energy Solution, meanwhile, rose 5.39 percent to 362,000 won. Samsung Biologics gained 1.42 percent to 1,573,000 won, and Samsung Electro-Mechanics ended up 0.15 percent at 1,318,000 won.

The Kosdaq rose. It ended up 11.39 points, or 1.42 percent, at 813.33.

The index swung between gains and losses during the session but held onto gains in the afternoon, staying above the 810 level. With selling pressure concentrated in KOSPI heavyweights, the Kosdaq showed a relatively steady performance.

In the Seoul foreign exchange market, the won-dollar exchange rate stood at 1,380.70 won, down 6.30 won from the previous session.