The Human Rights Lawyers Association has called on the caretaker government to repeal the emergency decree and use ordinary laws to handle the conflict on the streets.
The association said the situation does not warrant the decree as the survival of the nation is not under a grave threat.
Before the decree, the Internal Security Act (ISA) was in effect which allowed authorities to restrict the movement of anti-government protesters, the group said. The ISA covered the whole of Bangkok and surrounding provinces.
The emergency decree is not necessary and could lead to human rights violations as authorities can arrest suspects without charge and control media outlets, it said.
Media professional bodies, including the Thai Journalists Association and the Thai Broadcast Journalists Association, said the decree could give way to restrictions on media freedoms.
Democrat Party leader Abhisit Vejjajiva said the decree could empower the government to use force to contain the protesters.
The overall security situation will be more tense, he added.
Attacks including bombings and shootings have targeted the protesters. The protests are not violent as the blasts and shootings were not caused by the protesters.
Pichai Rattanadilok Na Phuket, deputy dean of Nida's School of Social and Environmental Development, said invoking the decree was tantamount to staging a self-coup to overthrow the constitution.
The sweeping powers to manage security afforded by the decree will set the country on a course toward dictatorship, he said.
Meanwhile, Pheu Thai spokesman Prompong Nopparit said he personally agreed with imposing the state of emergency.
The protesters have not staged peaceful rallies. They stopped prospective candidates from applying to run in the Feb 2 general election and seized state offices, he said.
The government also drew sympathy over the decree. Gan, an independent investment banker who asked that her surname not be disclosed, said the decree could help stem damage to government offices seized or surrounded by protesters. "The government needs to enforce a strict law," said Ms Gan, who joined a recent candle-lighting group calling for an end to the protests at Benjasiri Park in Bangkok.
Zakee Pitukkumpon, a lecturer at Prince of Songkla University's Institute for Peace, said decree or no decree, the government will not have any real power if parties such as the military do not cooperate. "Legally, the decree might allow the government a little help from the police. People do not feel as though the government can do anything.
"But the decree only worsens the image of the country, especially the tourism industry," said Mr Zakee.
Hara Shintaro, also of the Prince of Songkla University, said he disagreed with the application of any special law.
However, he recognised the government might be desperate to boost its powers to handle security.
Mr Shintaro said the government needs help from the army to keep order as the protesters appear to have taken the law into their own hands.