By 0340 GMT, the SET index was down 4.3 percent at 916.57, the lowest since September 2010.

The biggest energy firm, PTT , dropped 4.1 percent to 278 baht while top lender Bangkok Bank dropped 3.8 percent to 140 baht.

Bangkok-based dealers said the sell-off was led by foreign funds.

Southeast Asian stocks fell across the board, with Philippine shares down 4.2 percent and Malaysian stocks down 2.4 percent.