The Tourism Authority of Thailand (TAT) said the four major markets of Asean, South Asia, Oceania, and Africa are showing negative signs for bookings this month and next month.
TAT's office in Australia reported that hotel and tour bookings from Australian tourists are expected to grow by only 3.6% during the two months, down from a projection of more than 10%.
Sugree Sithivanich, deputy governor for marketing communications at TAT, said the political conflict reported in many media has caused some travellers to postpone their visits to Thailand.
He said regular travellers understood the situation in Thailand and avoided visiting protest areas.
If the problem ends before New Year, the number of international tourist arrivals in 2014 could reach the target of 28.01 million.
Thirty-one countries including Japan, China, India, Britain and the US have issued travel advisories at Level 2 for their citizens to avoid visiting areas near protests in the capital. Only Slovakia, Israel and Russia have raised their advisories to Level 3.
However, some Russian tourists, who form a major market, are changing their plans and visiting Pattaya, Hua Hin and Koh Samet instead of Bangkok.
Piyaman Tejapaibul, president of the Tourism Council of Thailand, said if travel advisories are raised to Level 5 and tourists are banned from visiting Thailand, the number of visitors will drop by 500,000 and lost tourism income will be 25 billion baht this year.
Despite the political tensions, the council has maintained its targets for 2013 of 26.1 million tourist arrivals and 1.18 trillion baht in revenue.
Ms Piyaman said bookings of air tickets and hotels by foreign travellers for January have been affected by the protests.
Asean tourists, particularly incentive travel groups, have changed their destinations to other regional countries, with 30% changing their destinations to Thailand's other provinces.
The meetings, incentives, conventions and exhibitions (Mice) segment has been significantly affected, with cancellations of about 30%.
Thirayuth Chirathivat, chief executive of Centara Hotels & Resorts, said more than 10 events have been cancelled due to the political unrest. Room bookings at its hotels in Bangkok stand at 60-70% for December and January.
"Although room reservations have had slight cancellations, we have to closely monitor the situation this week. If it ends up with more violence, we will see more cancellations," he said.
Accor Group said no cancellations had been made at its 24 hotels in Bangkok. The hotels have advised guests to avoid visiting protest areas.