Amnesty International has called for Thai authorities to restore freedom of expression and assembly rights, as well as drop charges against human rights defenders and activists who take part in peaceful campaigns.
In its annual human rights report released Wednesday, the international non-governmental organisation found military authorities extended their powers in the past year, excessively restricting rights and silencing dissent in the name of security.
"The number of people harassed, prosecuted, imprisoned and arbitrarily detained solely for the peaceful exercise of their rights escalated sharply," the report said.
The report documents the state of human rights in 160 countries and territories during 2015.
The full report can be downloaded at this link in PDF format.
In Thailand, since the May 22, 2014 coup, the National Council for Peace and Order (NCPO) has censored critical voices through restrictions imposed on the media and bans on political gatherings, Amnesty International Thailand chairman Chamnan Chanruang said.
Furthermore, the number of activists facing criminal charges for sedition rose sharply in the past year.
These include 14 New Democracy Movement activists and members of the Resistant Citizen group.
As civilians being tried in military courts, they are also denied access to a fair judicial process, Mr Chamnan argued.
Amnesty has repeatedly called for authorities to put an end to the practice, which violated international human rights standards.
Mr Chamnan also called for authorities to continue investigating cases of enforced disappearance, including high-profile cases such as human rights lawyer Somchai Neelapaijit and Karen land rights activist Porlajee "Billy" Rakchongcharoen.
The international NGO also raised concerns regarding the secrecy surrounding the judicial process in lese majeste cases in the country.
Section 112 of the Criminal Code -- the lese majeste law -- does not allow for the defendant to seek bail. The military courts which decide on many such cases also tend to give longer sentences than civilian courts, Mr Chamnan added.
In one case in 2015, a sentence of 60 years was imposed on one defendant.
He argued the law should be amended, to prevent it from becoming abused as a tool to harass or cause people harm.
Under the lese majeste law, anyone can file a lawsuit against the defendant, while in most criminal cases, only the damaged party is allowed to do so.
Since members of the royal family themselves cannot lodge complaints with the courts, Mr Chamnan suggested that a special committee be created instead.
More than 100 people have been arbitrarily detained by the military for so-called attitude adjustment sessions. Some had been called in repeatedly, at the military's pleasure.
They still have their rights restricted until this day, due to agreements they had to sign with authorities for their release, he said.
Government spokesman Sansern Kaewkamnerd played down the Amnesty report, saying it was no different to previous reports which criticised the government.
Maj Gen Sansern said the government led by Prime Minister Prayut Chan-o-cha has allowed people to express their opinions but the expression should be "proper".
Amnesty also submitted the report to the prime minister.