Protesters in Bangkok have surrounded several more government ministries as anti-government demonstrations continue.
They want prime minister Yingluck Shinawatra's government to resign, saying it is controlled by her brother - ousted former leader Thaksin Shinawatra.
Ms Yingluck invoked the Internal Security Act late on Monday, after two ministries were occipued.
"Everybody must obey the law and not use mob rule," she said. "If we can talk, I believe the country will return to normal."
The BBC reports the demonstrations began on Sunday. They are the worst in Thailand since 2010, when "red-shirt" opponents of the then-government occupied parts of the capital for two months.
More than 90 people, mostly civilians, were killed.
A government led by Ms Yingluck and the Pheu Thai Party was subsequently elected and since then Thailand has remained relatively politically stable.