The United States and other governments should pressure Thailand to revoke martial law and end press censorship which was imposed Tuesday, a leading human rights group said.
"The military's effective seizure of power and imposition of martial law across the country puts the rights of all Thais in jeopardy," said Brad Adams, Asia director at Human Rights Watch.
Army Commander-in-Chief General Prayuth Chan-ocha declared martial law early Tuesday in what he said was a bid to stem further protest-related violence and to help resolve the country's political stalemate that has dragged on for six months.
The law empowers the military to control all security matters, impose censorship of the media and enjoy a degree of impunity for its actions. Prayuth denied martial law was a coup.
On Tuesday, the military shut down 14 radio and satellite TV stations seen as under the control of political groups and stationed personnel in the country's main TV stations to enforce censorship of news.
"Press freedom has been the first casualty, but Thailand's friends around the world need to speak out to prevent the assault on other basic rights," Adams said.
The US and European Union on Tuesday issued cautionary statements on the imposition of martial law, appealing to the military to respect democratic principles and speed up the holding of a general election.
Thailand has been without a parliament since December 9. The results of snap election held February 2 were annulled by a court ruling, and no new polling date has yet been confirmed.
"The priority now is to set a clear timetable for early elections and establish as soon as possible a fully functioning government with democratic legitimacy," the EU said in a statement from Brussels.
To date, the Thai military has not applied the full force of martiallaw.
No curfew has been imposed in Bangkok, where most of the protests have been staged, and only a few troops have been stationed on the streets.
So far, the anti-government and pro-government street movements have stayed at their rally sites, as ordered by the military, but if they renew street protests the army is empowered to outlaw the assembly of more than five people and detain people for up to seven days without charge.
Under martial law, any abuses by soldiers of human rights will be tried by military courts, not civilian ones.
"Civilian rule needs to be restored and elections scheduled so that the Thai people can decide who governs the country," Adams said. "Every minute martial law is in effect, the rights of Thais are being undermined."