"We will step up our rallies to counter the emergency decree,'' he told the crowd at his Pathumwan rally site.
Mr Suthep said he will lead protesters to challenge the decree, arguing that the caretaker government's decision would draw more people onto the streets of Bangkok to oust caretaker Prime Minister Yingluck Shinawatra.
"We will defy them all,'' he said.
The People's Democratic Reform Committee (PDRC) leader attacked the caretaker government for having no grounds to declare a state of emergency, claiming that the invocation of the law showed the caretaker administration was being cornered by protesters.
Anti-government rallies have been held in an open and peaceful manner, so do not substantiate the decision to invoke the decree, he added.
"There are no national threats. There are only threats to them,'' he said, referring to the caretaker administration.
The former Surat Thani MP stressed the importance of remaining patient as protesters brace for more pressure from security officials once the decree is enforced.
PDRC core member Issara Somchai said further protests to shut down government and state offices will be underway on Wednesday.
The emergency decree will be enforced in all areas of Bangkok and part of Nonthaburi, Pathum Thani and Samut Prakan provinces.
Caretaker Labour Minister Chalerm Yubamrung will oversee the enforcement of the decree through the centre for peacekeeping. He will be assisted by national police chief Pol Gen Adul Saengsingkaew.