BANGKOK (Reuters) - Thai shares touched two-week lows on Monday, underperforming other Southeast Asian stock markets, as investors trimmed holdings in shares of tourism-related and consumer companies amid concerns about domestic political tensions.

The key SET index was down 0.6 per cent at midday, falling almost 1 per cent at one point to 1,348.24, the lowest since March 10. Top losers included shares of retailer CP All and airport operator Airports of Thailand.

Sentiment is likely to remain weak as investors await the Election Commission's conclusion on a new election expected later in the day following the Constitutional Court's ruling on Friday that the Feb 2 election was invalid.

Supporters of Thai Prime Minister Yingluck Shinawatra said over the weekend they will take to the streets as moves to impeach her gather pace, raising the spectre of confrontation with protesters who helped scupper a February election she had been expected to win.

"The market is likely in for a volatile day today on further foreign net selling as the baht weakened for the fourth straight day," said a strategist at broker Krungsri Securities.

Other markets in the region followed Asian stock markets higher, with Singapore, the Philippines and Vietnam each climbing almost 1 per cent while Malaysia and Indonesia both posted smaller gains.

Market breadth was relatively cautious after the China HSBC flash manufacturing purchasing managers index (PMI) fell to an eight-month low in March.

Singapore-based NRA Capital expects profit taking to cap gains on the city state bourse.

"Attempts to rally will likely be met with profit taking as liquidity concerns seems to be increasingly important in China and Chinese investors reportedly liquidating overseas assets in a hurry," the broker said in a report.