Expressing a strong opinion for or against the draft constitution ahead of the Aug 7 referendum could bring a jail term of up to 10 years under the Referendum Act now in effect.
Other penalties include a fine of up to 200,000 baht and the revocation of voting rights for 10 years, according to the law published in the Royal Gazette on Friday.
While Section 7 of the law gives a person a freedom to express and propagate opinions honestly and in line with the laws, another section bans expressions that authorities deem "inconsistent with the truth".
Section 61 says that text, pictures or sounds that are "inconsistent with the truth or in a violent, aggressive, rude, inciting or threatening manner aimed at preventing a voter from casting a ballot or vote in any direction or to not vote shall be considered disrupting the referendum".
The prohibition applies to comments disseminated through newspapers, radio and TV broadcasts, as well as electronic channels or other means.
While individuals could face up to 10 years in jail, if the offences are committed by a group of more than five people, each one will face imprisonment of 1-10 years, a fine from 20,000 to 200,000 baht and a 10-year revocation of voting rights.
The law was released just hours after the United Nations human rights commissioner spoke out against the military government's curbs on dissent.
“An open and dynamic public debate on the draft constitution would foster national unity, strengthen the legitimacy and acceptance of the constitution and provide a sense of collective ownership,” said Zeid Ra'ad Al Hussein.
“I urge the government to actively encourage, rather than discourage, dialogue and engagement on the draft.
"Extending the military's powers is not the answer to rebuilding Thailand's political landscape," he added.
Local rights activists have branded the law a tool to suppress dissent and to gag media and academics because interpretation is subjective and discretionary. As well, the penalty is deemed out of proportion to the offence.
The Foreign Ministry has countered that "public order and social harmony" are needed in order for the government to complete its work and that’s why it had enacted various laws.
"These laws do not impinge on general freedom of expression — which we believe to be a fundamental element of a democratic society — as long as such expression does not undermine public order and social harmony," it said in a document distributed to the media on Friday.
"In fact, the government has been receptive to all views regarding the current process of reconciliation and reform."
The two-question constitutional referendum will be held from 8am to 4pm on Aug 7. Thai nationals aged 18 or more on that date are eligible to cast ballots. The sale and distribution of alcohol will be banned from 6pm on Aug 6 to midnight of Aug 7.
Gambling on the result is prohibited, as are the use of force, threats and bribery to sway voters. Pollsters also cannot conduct surveys seven days before and on the referendum date.
An employer who does not allow employees time to vote will face two years in prison and a fine of 40,000 baht, or both. (Section 58)
The law calls for ballots will be counted openly at polling stations without delay. Only ballots marked with an X in one of the boxes for the constitution question will be accepted. No other marks can be made.
Destroying or damaging a ballot could result in a jail term of up to one year and a fine of up to 200,000 baht.
Taking photos of a filled ballot is illegal (Section 60) and will be punished with not more than five years in jail and or a fine from 20,000 to 100,000 baht.
Photographing ballots with mobile phones has been widespread in recent general elections, as the pictures offered proof to politicians who bought votes that their money was properly spent.