The KOSPI tumbled more than 5 percent on heavy foreign selling. It broke below the 6,000 level during the session and slid to the 5,900s, but pared some losses in the afternoon to narrowly recover 6,000.

On Oct. 13, the KOSPI closed down 348.89 points, or 5.46 percent, at 6,035.35. It opened down 198.16 points, or 3.10 percent, at 6,186.08 and quickly extended losses. It fell below 6,000 for the first time since it regained that level on July 31.

Buying at low prices came in during the afternoon, trimming losses. The index held above 6,000 on a closing basis.

Foreign investors led the decline, posting net sales of 4.5128 trillion won. Retail investors net bought 2.2271 trillion won and institutions net bought 1.6903 trillion won.

Among the top stocks by market value, chipmakers saw the biggest declines. Samsung Electronics closed down 7.39 percent at 232,000 won, and SK Hynix plunged 10.09 percent to 1,443,000 won.

SK Square slid 12.91 percent, the biggest drop among the top market-cap names. Samsung Electro-Mechanics fell 8.34 percent, Samsung Life dropped 4.30 percent, Hyundai Motor slipped 1.56 percent and KB Financial fell 0.25 percent.

LG Energy Solution rose 3.02 percent, while Samsung Biologics added 0.42 percent.

The KOSDAQ closed down 12.16 points, or 1.43 percent, at 837.03, a smaller drop than the KOSPI.

The won-dollar exchange rate was 1,344.40 won, up 8.40 won from the previous session.

The market focused on signs that selling pressure in Samsung Electronics is intensifying despite its record-high quarterly results.

Kim Seok-hwan (김석환), a researcher at Mirae Asset Securities, said "foreigners have already net sold about 200 trillion won in the KOSPI this year, and selling has continued into October," adding that "the market is placing greater emphasis on a lack of new buying to absorb foreign selling than on improving earnings."