The decree, implemented after Prime Minister Yingluck Shinawatra met with officials this afternoon, is designed to give authorities a greater say over what anti-government protesters do next.
The decree limits the number of people who may gather in one spot and can be used to censor media coverage. Suspects can also be more easily detained.
The imposition of the decree follows eight days of the Shutdown Bangkok campaign that has paralysed parts of the capital and led to cancellations by many tourists.
In Phuket and across the southern provinces, many government offices have also been closed since the campaign began last Monday.
Khun Yingluck called a national election for February 2 but it seemed unlikely to deliver enough candidates to become a legitimate result without government intervention.