By Amy Sawitta Lefevre and Andrew R.C. Marshall

BANGKOK (Reuters) - From the question of Britain's place in Europe to the choice of New Zealand's flag, referendums worldwide are usually preceded by unbridled debate. Not so under Thailand's junta.

Many Thais are chafing under strict new regulations governing discussion ahead of an Aug. 7 referendum on a military-backed constitution.

The junta that seized power in a May 2014 coup has already threatened to jail anyone campaigning for or against the constitution, which critics say entrenches the military's political influence.

The 14 rules, which were issued by the Election Commission and formally became law on Monday, make even well-meaning discussion risky, say academics and experts.

Under the regulations, Thais must express their opinions with "polite words ... without distorting the facts".

"Rude, aggressive, or intimidating" interviews with the media are banned. So is organizing a panel discussion "with intent to incite political unrest".

Also forbidden are "T-shirts, pins and ribbons" that encourage others to campaign.

Violators can be jailed for up to 10 years. Dissenters in military-run Thailand often receive lengthy prison sentences under draconian laws on computer crime and royal defamation.

The referendum will be a test of the junta's popularity and a potential flashpoint in a turbulent political scene, say analysts. The military government has promised an election by mid-2017, even if the constitution is rejected.

Groups of all political stripes have denounced the draft constitution as undemocratic, with one major political party urging supporters to vote "no".

"To express opinions using reason. Is that so hard to understand?" Prime Minister Prayuth Chan-ocha snapped at a reporter who asked about the new rules on Tuesday.

Prayuth has ruled largely unchallenged but anti-junta activists have recently staged small but frequent protests.

Nine activists were jailed last week on charges of sedition and computer crimes. Two face additional charges of insulting the monarchy.

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Phubed Pisanaka, a recent law graduate who comments on the government on his Facebook account, said the rules would make him more careful.

"I have to think twice about what I post and share now," he said.

Others remain defiant.

"I'll keep expressing my opinion even though I could be criminalized," said Kornkritch Somjittranukit, a contributor to Thailand's online publication Prachatai.

"If thinking differently is a crime, living inside or outside of jail is practically the same," he said.

(Additional reporting by Panarat Thepgumpanat and Patpicha Tanakasempipat; Editing by Robert Birsel)