Thailand's biggest investor Japan on Tuesday expressed "grave concerns" after the army imposed martial law.
After almost seven weeks of anti-government protests, generals ordered forces onto the streets of Bangkok and troops were positioned at television stations as the army said the media would be censored.
"We have grave concerns about the situation in Thailand," Japan's chief cabinet secretary Yoshihide Suga told reporters in Tokyo. "We once again strongly urge all parties concerned to act in a self-restrained manner without using violence."
Private-sector think-tank Teikoku Databank said in February nearly 4,000 Japanese firms operate in Thailand, with investments the Bank of Thailand said were worth $6.89 billion in 2013 -- half of the total inward investment.
That figure is more than the next three biggest investors combined -- the United States, Britain and the Association of Southeast Asian Nations (Asean).
Thailand has become increasingly important for Japanese firms as they shift operations from home to counter high wages and an overvalued yen and to mitigate the effects of natural disasters on the supply chain.
Car giant Honda said political instability was leading it to reconsider a second assembly plant it is hoping to go online in April 2015.
Spokesman Teruhiko Tatebe said: "We are watching the political situation in order to decide to go ahead with the plan (to start operations) or not. If the political situation improves, we may complete the factory and start production."
And Toyota, the world's biggest automaker, also said it was watching events carefully, but added all three of its plants were operating normally. "The morning shift started as per usual at all plants. A decision concerning the evening shift will be made based on the situation," a spokesman said.