Prime Minister Yingluck Shinawatra has called February elections following weeks of opposition street protests. But the demonstrators have vowed to block the vote, which they fear will only prolong the political dominance of her billionaire family.
The protesters say they will occupy the capital from January 13 until they win their battle to topple the government.
They plan to set up stages around the city, preventing officials from going to work and cutting off power and water to state buildings.
"Our goal is to prevent any violence or clashes," he said.
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Thai stocks and the baht currency have fallen sharply on concerns that the deepening crisis will scare off foreign tourists and international investment.
Singapore Airlines has already cancelled 19 flights to Bangkok between 14 January and 25 February.
Bangkok city authorities have instructed 146 schools to close on Monday because of the shutdown.
Officials say the government is ready to declare a state of emergency if needed to deal with any unrest, following several outbreaks of street violence in which eight people, including a policeman, have been killed and hundreds wounded.
Thailand has been periodically shaken by political bloodshed since Yingluck's older brother Thaksin Shinawatra was toppled by royalist generals in a coup seven years ago.
The demonstrators, who largely comprise southerners, royalists, middle-class Thais and urban elite, accuse the billionaire tycoon-turned-politician of corruption, and say he controls his sister's government from his self-exile in Dubai.
They want an unelected "people's council" to run the country to oversee vague reforms, such as an end to alleged "vote buying" through populist policies, before new elections are held in around a year to 18 months.
The recent civil strife is the worst since 2010, when more than 90 people were killed in a bloody military crackdown on pro-Thaksin Red Shirt protests under the previous government.
Ex-deputy premier Suthep Thaugsuban was due in court today to face a murder indictment over those deaths, but he asked for another postponement because he is leading the current protests.