Vietnam's benchmark VN Index dropped 5.14 percent at the break on Monday, marking its biggest fall since May 2014, with banks and energy stocks leading most shares down on declines in U.S. stocks and tracking global oil prices lower.
The index closed the morning session at 527.73 points, on track for the worst single-day percentage loss since May 8, 2014.
As many as 99.72 million shares changed hands, compared with the five-day average of 119.78 million, Thomson Reuters data showed.
"The fall is due partly to the drop in U.S. stocks last Friday", said Nguyen The Minh, Retail Research Manager at VietCapital Securities.
Fears of a China-led global economic slowdown drove Wall Street to its steepest one-day drop in nearly four years on Friday and left the Dow industrials more than 10 percent below a May record.
Decliners outnumbered gainers by more than 13 times.
Vietcombank, the country's top listed lender by market value, shed 6.13 percent, while PetroVietnam Gas lost 6.53 percent.
Vietnam devalued the dong by 0.99 percent and raised the dollar/dong trading band to 3 percent from 2 percent on Aug. 19, underscoring concerns a weaker yuan could further inflame its bloated trade deficit.
Oil prices resumed their downward trend on Monday, with Brent and U.S. crude oil futures hitting fresh 6-1/2-year lows, as investors fretted that a slowing Chinese economy will lead to weaker demand amid a global supply glut.
Other blue chips on Vietnam's main exchange also dropped, including dairy products maker Vinamilk, Vietnam's biggest firm by market capitaliZation, with a 5.94-percent loss and food producer Masan Group that tumbled 6.63 percent.
Following is a snapshot of the VN Index at midday (0431 GMT).
VN Index: 527.73
PREV. CLOSE: 556.30
% CHANGE: -5.14
Change (%) 1-mnth: -9.781
Change (%) 3-mnth: 0.677
Change (%) 1-year: -9.241
52-week high: 644.56 (3-Sep-14)
52-week low: 513.06 (17-Dec-14)