Bangkok: Fugitive former prime minister Thaksin Shinawatra has broken two years of silence to compare Thailand's military government that seized power in a 2014 coup to tyrants.

In a tweet Mr Thaksin quoted the 1770s French lawyer Charles-Louis de Secondat, Baron de Montesquieu as saying "There is no crueller tyranny than that which is perpetrated under the shield of law and in the name of justice."

Mr Thaksin, a communications mogul who lives in exile in Dubai, made the comment five days after his younger sister Yingluck Shinawatra fled Thailand ahead of a court verdict that could have seen her jailed for 10 years.

Ms Yingluck is believed to have joined her brother in Dubai.

Thailand's prime minister Prayuth Chan-ocha, a former army general who led the coup to topple Ms Yingluck's government, ridiculed the tweet, saying "Let him do it. He has tweeted it. What would you do? If you want to believe him, it depends on you. Think about it. Use your brain."

Ousted by a military coup when he was prime minister in 2006, Mr Thaksin has largely kept a low profile while living in self-imposed exile, after also fleeing Thailand almost a decade ago to escape a jail sentence on corruption charges.

Both Ms Yingluck and Mr Thaksin claim they are victims of a witch-hunt by Bangkok's military, royalist, middle class and elite establishment.

Analysts say Ms Yingluck's departure marks an end of the Shinawatra family era in Thailand during which its political machine won the past five general elections.

Mr Prayuth and authorities have denied widespread speculation that Ms Yingluck's departure was facilitated by government as part of a deal with her family.

The military has accused Mr Thaksin of arranging the escape but have not presented any evidence. 

Authorities say they have no idea how Ms Yingluck managed to flee while being monitored by security forces ahead of the verdict on a charge relating to her alleged mishandling of a subsidy scheme that benefited rice farmers when she was in power.

She was not accused of corruption, although her former commerce minister has been sentenced to 42 years' jail in a corruption case linked to the subsidy scheme.

Ms Yingluck had already been barred from holding public office, being fined $US1 billion and had her 16 bank accounts frozen over the subsidy scheme that caused billions of dollars losses to the state. 

The military government has pledged to allow the country to return to democracy but has not a set a date for elections.