The SET Index lost 40.68 points to close at 1,388.40 on turnover of Bt34.7 billion. Foreign investors were unloading shares, with their sales outpacing purchases by Bt777 million.
In the morning, the index lost 32.05 points or 2.24 per cent, pushing the composite index to fall below the 1,400-point psychological level at noon. The index at noon was 1,397.03 points, on turnover of Bt15.5 billion.
Brokers commented that spreading protests in Bangkok and provinces against the amnesty bill are a big concern for stock investors.
"Growing political tensions triggered by the controversial amnesty bill are likely to dampen market sentiment further in the first half of November," said Tisco Securities.
Trinity Securities noted that if there is no violence outside the Parliament, the SET Index may see a chance to test 1,480 points again.