While martial law has brought calm to Thailand's politics, the military regime's stifling of dissent precludes a frank dialogue on the future of the south-east Asian country, the Brussels-based group warns in a major report.

"More than ever, the society is riven across regional, ethnic and quasi-ideological lines, by deep income inequality and by a difficult relationship between Bangkok and its peripheries," it says.

The report warns that by curbing the power of elected representatives in favour of appointed officials, the coup makers risk yet another round of violent conflict.

The regime that calls itself the National Council for Peace and Order insists it had no option to mount Thailand's 12th coup after months of sometimes violent upheaval.

After initially promising elections in 2015, the regime led by former army general Prayuth Chan-ocha has indicated it intends to stay in power for longer as an inevitable royal succession looms in the country that venerates the king.

A large crowd of well-wishers is expected to witness a palace audience on Friday of King Bhumibol Adulyadej, the world's longest reigning monarch, on his 87th birthday.

The International Crisis Group report says at the heart of Thailand's turmoil is not only a political struggle but disagreement over what constitutes legitimate authority, with some regarding the popular ballot as paramount and others regarding majoritarianism as another form of tyranny, requiring strong checks and balances by the establishment.

"In the background, a looming royal succession – prohibited by law from being discussed – adds to the uncertainty," the report says.

The report says that for the regime to establish a durable democracy it must encourage the development of a national dialogue, provide for meaningful political participation of all and reach out to particularly those in the north and north-east who believe they have been disenfranchised by the Bangkok establishment.

The group's chief policy officer Jonathan Prentice says Thai society is both deeply divided and – now – accustomed to having a political voice.

"Stability will remain elusive unless Thailand forges a political path in which all Thais respect the majority vote and see their own concerns acknowledged," he says.