The business sector is worried that tension on the streets of Bangkok could undermine trade and investment, Supawut Saichua, managing director for research of Phatra Securities Public Company Limited, said.

Mr Supawut said some foreign investors did not fully understand the evolving political situation in the wake of yesterday's Pitak Siam rally.

The investors did not anticipate any violence during the anti-government demonstration, Mr Supawut said, but they had seen reports of police using tear gas to keep the protesters from advancing to the Royal Plaza, where the political demonstration was being held.

He said last week's landmark visits to Thailand by US President Barack Obama and Chinese premier Hu Jintao heightened the country's image and appeal to international investors.

But Mr Supawut warned that the country's political situation was now in a precarious state, and this could potentially deter future foreign investment in Thailand.

Anusit Phuwaset, former chairman of the Lampang chamber of commerce, said businesses were closely monitoring the anti-government movement.

Some businesses, fearing a protracted protest or political violence, might put their investments on hold, damaging the country's economy, he said.

Sa-nga Ruengwattanakul, chairman of the Khao San Road Traders' Association, said entertainment venues on on the popular tourist strip reported a 25% drop in revenue on Friday night, the eve of the Pitak Siam rally.

Khao San Road is a short distance from yesterday's rally site on Ratchadamnoen Nok Avenue.

Mr Sa-nga said more than 1,000 traders occupy the Khao San Road area and they generate 800 million baht a night on average.

Friday trade slumped by almost 200 million baht as more than 1,000 police officers were deployed in the area to maintain security, he said.