Fugitive former Thai prime minister Thaksin Shinawatra hit out at "tyranny" on Wednesday via a cryptic tweet quoting a French Enlightenment thinker, in his first comments since his sister fled overseas.

Yingluck Shinawatra, who like her brother was ousted from the post of prime minister by a coup, dramatically disappeared before a court was about to deliver judgement in a criminal negligence trial.

Party and junta sources say she went to join her brother, who lives in self-exile in Dubai.

On Wednesday Thaksin broke a long public silence, issuing tweets in both English and Thai that quoted 18th century French philosopher Charles de Montesquieu.

"Montesquieu once said 'There is no crueller tyranny than that which is perpetuated under the shield of law and in the name of justice'," he wrote on his Twitter account.

It is the first time Thaksin has used Twitter in more than two years, in an apparent swipe at the military rulers who forced him out of office in 2006 and toppled his sister's administration in 2014.

The message quickly went viral on Thai social media and was retweeted more than 4,000 times within five hours. 

One striking endorsement came from Princess Ubolratana, the oldest daughter of the late King Bhumibol Adulyadej.

Thaksin's youngest daughter Paetongtarn reposted her father's tweet on her Instagram page under which Princess Ubolratana wrote: "I agree!!! Su Su". (Su means "fight" in Thai). 

Yingluck faced up to 10 years in prison and a lifetime ban from politics had she been convicted by the Supreme Court last Friday. But she failed to appear, forcing the court to delay its ruling and issue a warrant for her arrest.

Thaksin fled overseas in 2008, two years after the coup that toppled him, to escape a graft conviction he says was politically motivated.

"That tweet after so long reflects Thaksin's anger," said Pavin Chachavalpongpun, a former Thai diplomat and an academic at Kyoto University.

"I suspect Yingluck and Thaksin will continue to find a political space in Thailand (from overseas)... they want to take revenge" on their enemies, he added.

The Shinawatra political dynasty began under Thaksin in 2001 with a series of groundbreaking welfare schemes that won them votes and the loyalty of the rural poor.

But their popularity rattled the royalist and army-aligned elite, who assailed successive governments linked to the clan with coups, court cases and protests.

The period since the first coup has been dubbed the "Lost Decade", with frequent deadly street protests, short-lived governments and the return of entrenched military rule.

Source: AFP