The Bangkok shutdown that began on Monday has affected motorists above all but also hotels, tourism, information technology and domestic consumption.
The Association of Thai Travel Agents reports tourism is set for further damage if the rallies continue.
President Susdivachr Cheewarattanaporn said the ATTA would like to see the protests end as fast as possible.
"Our big concern is that foreign tourists are trying to avoid Bangkok now," he said.
Reservations for the upcoming Chinese New Year holiday now stand at 40-50%, compared to 100% in a normal situation.
Chanin Donavanik, the chief executive of Dusit International, said the prolonged political turmoil has already dampened business at the Dusit Thani Hotel on Silom Road, a key protest area.
Occupancy at the Dusit Thani is 20%, down from 80% a year ago.
Thailand's information technology sector is bracing for a sharp contraction of 25% in sales of consumer IT products in the first quarter if the conflict drags on.
Such a plunge would lead to the industry's deepest contraction, said Pathom Indarodom, the chief executive of Arip Plc, organiser of the Commart Thailand IT fair.
The Federation of Thai Industries (FTI) said the ongoing clash could crimp international orders and further slow the domestic economy.
The FTI also announced the Thai Industries Sentiment Index for December fell to a 25-month low of 88.3, down from 90.3 in November.
Chairman Payungsak Chartsuthipol said the Thai economy still has strong growth potential this year but only if the political situation is settled.
Bangkok Post reporters summarise the impact of the Bangkok shutdown as follows:
Tourism
The ATTA said the rallies are weighing heavily on the Asian market, especially tourists from Singapore and Hong Kong who normally come to Bangkok but are avoiding the capital now.
There should not be a significant effect on Russians and other long-haul visitors, as they mostly visit Phuket and Pattaya.
"We have received a lot of feedback from members in the tourism sector reporting a 30% drop in tourist numbers," said Stanley Kang, chairman of the Joint Foreign Chambers of Commerce in Thailand.
Information technology
Arip's Mr Pathom said the IT market must rebound by June, or else many small and medium-sized IT retail shops will be forced to close.
Jarit Sidhu, a senior analyst at IDC (Thailand), said the political unrest has already hit the local IT market hard, reflected in 2013's fourth-quarter sales shrinkage of up to 15%. He said the Bangkok riots in 2010 led to a 30% contraction of the IT market.
Stock exchange
The Thai stock market has so far been unaffected by the Bangkok rallies.
Montree Sornpaisarn, the chief executive of May Bank Kim Eng Securities, said most firms adopted a more flexible approach allowing staff to work from home.
Investors can place trading orders using the telecommunications network or through the trading houses of securities firms around Bangkok.
"Foreign investors will return to the Thai stock market when the prices become cheaper," said Mr Montree.
Logistics
Worawit Charoenwatanaphan, president of the Thai Transportation and Logistics Association, said most firms are transporting goods by 6am.
"Thais are being used as a tool by both the Democrats and Pheu Thai, and Thailand should reform its politics and eradicate corruption from the political system," he said.