Protest leader and former Democratic member of parliament Suthep Thaugsuban promised at least 1 million anti-government protesters would gather near Democracy Monument by the end of Sunday.
"I arrived yesterday in a convoy of about 40 buses from Krabi province," said Wilawan Kayathan, 53, a Thai Muslim. "We want to get Yingluck out and bring in a new government. This government is untrustworthy."
"This country needs rule of law, otherwise society is weak," said Anand Pomsuksan, 49, a factory worker who joined the rally. "I don't like the Democrats either but they are more trustworthy than the Pheu Thai Party."
Due to safety concerns over the growing crowd, the Bangkok Metropolitan Administration ordered the closure of 24 schools in Dusit, Phra Nakhon and Pomprap Sattruphai districts Monday and Tuesday, the Bangkok Post reported.
Yingluck came under fire after backing an amnesty bill that would wipe clean the slate for those convicted of offenses arising from protests against a 2006 military coup.
Critics say the bill is actually intended as a means of allowing fugitive former Prime Minister Thaksin Shinawatra, the current prime minister's brother, to return home.