By Simon Webb and Aukkarapon Niyomyat
BANGKOK (Reuters) - Former Thai Prime Minister Thaksin Shinawatra on Thursday criticized a military-backed draft constitution as a "folly" that would perpetuate the power of the ruling junta and make it impossible for future elected governments to rule.
The country will decide in a referendum on Sunday whether to accept the constitution in the biggest test of public opinion since the generals seized power in 2014. A vote in favor of the charter could give the military a permanent role in overseeing economic development and politics.
As about 3,000 students, civil servants and military cadets gathered in Bangkok to urge people to vote, Thaksin said in a statement emailed to Reuters the charter was a "nightmare of contradiction and confusion".
"The drafters ... created a constitution for the 'continuity' of the absolute power of the present coup makers to continue even after the new constitution is proclaimed," he said in response to a question about his opinion on the referendum.
The ruling military council has said the charter will pave the way for a general election in 2017, ensure clean politics and end more than a decade of political turmoil since Thaksin, his allies and their rural supporters challenged the royalist and military establishment.
Thaksin, a former telecommunications tycoon who won the loyalty of legions of poor voters with populist policies, was toppled in a 2006 coup. He lives in self-exile to avoid a graft conviction he says was politically motivated.
Thailand's two biggest political parties, one of which is loyal to Thaksin, oppose the constitution because they say provisions are designed to ensure military supervision of politics.
The constitution would put so much power in the hands of bodies tasked with acting as counter-balances to governments that it would make Thailand ungovernable, Thaksin said.
"I predict that, even if the new government receives perfect endorsement from the present regime, it will find it impossible to manage the Thai economy or administer the country under those proposed conditions," he said.
The junta, known as the National Council for Peace and Order, has said it wants to return Thailand to democracy but to ensure politicians put the people's interests first.
However, Reuters interviews with senior military officers belie those claims and show that the military's ambition is to make future coups unnecessary by weakening political parties.
Under the proposed charter, which would replace one torn up following the 2014 coup, a junta-appointed Senate with seats reserved for military commanders would check the powers of elected lawmakers.
The vote comes against the back-drop of concern about the health of King Bhumibol Adulyadej, 88. The military has for decades invoked its duty as defender of the deeply revered monarch to justify its interventions in politics.
The junta has promised a general election in 2017 if the constitution is approved. A 'no' vote could usher in more uncertainty.
Prime Minister Prayuth Chan-ocha, who is also junta leader, has said he would come up with another charter if this one is rejected.
Crowds gathered at central Bangkok's Royal Plaza on Thursday as cars bearing banners urging people to vote set off around the capital.
The government has banned provocative criticism of the charter and detained dozens of activists. Referendum rules also ban anyone from campaigning for either side, so people carried neutral signs, urging voters to cast ballots.
About 50 million Thais are eligible to vote in the referendum and the Election Commission has said it expects an 80 percent turnout.
The junta has broadcast songs and television programs to drum up enthusiasm.
"Everyone in the country ... must come out to exercise their right to vote," said Supachai Somcharoen, chairman of the Election Commission.
"This is the big day."
(Additional reporting by Panarat Thepgumpanat Patpicha Tanakasempipat, Noah Browning in DUBAI; Writing by Amy Sawitta; Editng by Paul Tait, Robert Birsel)