The two major opposition parties, Pheu Thai and Move Forward, denounced Prime Minister Prayut Chan-o-cha's decision to intensify the government's response to protest violence, saying it would not solve any problems.
Pheu Thai spokeswoman Arunee Kasayanont said on Thursday that at a time of extreme political conflict, heavy law enforcement might not be the right solution. The science of politics should also be applied.
She advised the government to allow stakeholders to take part in solving national problems.
Ms Arunee said the prime minister's announcement did not specify clearly which laws would be enforced against protesters, and this ambiguity could worry demonstrators.
She was concerned that the statement may only trigger more violent protests.
Natcha Boonchai-insawat, spokesman of the Move Forward Party, said the prime minister's statement reflected the stubbornness and cruelty of the government, while the protest movement's actions, although escalating, remained symbolic.
Shops remained open and electric trains continued to run despite the big rallies, because the operators knew that the demonstrators were not rioters, he said.
The prime minister's resignation could end the matter, he said.