At 3.08pm, the index lost as much as 31.40 points or 2.19 per cent, on turnover of Bt23.5 billion.
Stock analysts said that the ISA brought about fears that political uncertainties might heighten. Aside from political uncertainties, the market is under pressure from the unclear direction of the US Federal Reserve's easing on quantitative easing.
The Cabinet today announced the enforcement of Internal Security Act in three districts of Bangkok in preparation for anti-government protests.
The ISA is issued on Dusit, Phranakhon, and Promprab Satrupai districts from August 1 to 10.