The warnings cast further gloom over a vital pillar of the Thai economy that has already been hard hit by the twin effects of earlier and the global economic downturn.
On Monday, with a state of emergency declared, tourism industry bosses pleaded with the government to bring an early and lasting end to the political crisis that has wracked the country for months.
"Who will want to come to Thailand now?" Apichart Sankary, president of the Association of Thai Travel Agents (ATTA), told AFP news agency.
"We have nothing to lose anymore, no tourists will come now anyway so we need to clear everything as soon as possible. We cannot have this situation go on and on."
Apichart said the latest clashes put paid to hopes that Thailand would attract 14 million tourists this year, adding that if the political turmoil was not resolved by next month, Thailand may struggle to attract 10 million.
|The tourism industry lost millions from the closure of Bangkok's airports last year [EPA]
That could threaten hundreds of thousands of jobs that directly or indirectly depend on the tourist trade.
"We don't want a state of emergency but if they don't, the mobs will keep coming and these troubles will never end," Apichart said.
Tourism, which accounts for about six per cent of Thailand's GDP, was badly-hit late last year when yellow-shirted anti-government protesters blockaded Bangkok's two main airports for a week just as the Christmas high season began.
With the global downturn now hitting the rest of the Thai economy, as well as slowing the global tourist trade as a whole, experts say Thailand's travel industry stands to lose billions of dollars unless calm can be restored soon.