The KOSPI, which jumped more than 6 percent the previous day, slumped more than 5 percent in early trading and fell below the 7,000 level. Selling pressure widened, centered on large semiconductor stocks, and a selling sidecar was triggered on the main board.

As of 9:07 a.m. on July 16, the KOSPI was down 323.28 points, or 5.57 percent, from the previous session at 6,874.13.

The index opened 323.91 points, or 4.45 percent, lower than the previous close at 6,960.50 and then immediately slipped below 7,000.

The Korea Exchange triggered a selling sidecar at 9:10:26 a.m., a temporary halt to the effectiveness of program sell orders on the main board.

At the time it was triggered, the KOSPI was down 353.01 points, or 4.85 percent, from the previous session at 6,931.40. The effectiveness of program-trading sell orders was suspended for 5 minutes.

The latest trigger brought the total number of sidecar activations on the main board this year to 37. Buying sidecars were triggered 19 times and selling sidecars 18 times. Sidecars operated on 8 of the past 12 trading sessions.

In the main board market, individuals were net buyers of 213.4 billion won. Foreigners and institutions were net sellers of 127.4 billion won and 80.2 billion won, respectively.

Among the top stocks by market capitalization, semiconductor shares posted notable declines. Samsung Electronics was trading at 263,750 won, down 15,750 won, or 5.64 percent, from the previous session.

SK Hynix fell 169,000 won, or 8.12 percent, to 1,913,000 won, slipping back below the 2 million won level. SK Square fell 9.19 percent and Samsung Electro-Mechanics dropped 8.35 percent.

Hyundai Motor and Samsung Life Insurance were also weaker, down 3.00 percent and 1.48 percent, respectively. By contrast, LG Energy Solution rose 2.84 percent, KB Financial Group added 1.54 percent and Samsung Biologics was up 0.07 percent.

The Kosdaq was also down 16.26 points, or 1.96 percent, at 813.17 at the same time. It had surged more than 5 percent the previous day to reclaim the 820 level but returned to declines a day later.

The won was trading at 1,486.60 per dollar, up 1.90 won, or 0.13 percent, from the previous session.