Former prime minister Yingluck Shinawatra has warned that the criminal negligence case against her would make future leaders rethink any policy to help Thailand's citizens.

In her first testimony since being brought to court for alleged mismanagement over her government's rice pledging scheme, she urged people not to judge a policy by its financial gains. "No public policy benefits you financially," she said. "If you measured public policy only by financial benefits, no government would want to make the decision."

She is accused of dereliction of duty over her administration's rice pledging policy, which allowed farmers to sell their rice to the state at about 50 per cent more than the market price.

The scheme was a flagship programme of her Puea Thai party, which draws considerable support from its rural base in the north and north-east of the country.

Yet it ran aground in early 2014, after accumulating more than 17 million tonnes of rice stocks over three years. It also drew the ire of the Bangkok middle class, who felt that their taxes were being squandered on "populist" schemes laden with corruption.

If Yingluck is convicted, she could be jailed for up to 10 years.

As there are 41 other witnesses to examine in this case, the trial is expected to last until next February.

More than 200 supporters greeted the former leader outside Bangkok's supreme court yesterday, shouting "Yingluck fight, fight!" as they surged forward to hand her roses.

The military government has indicated that it intends to pursue civil damages against her to claw back what it considers as losses from the scheme.

Mr Panada Disakul, a minister in the prime minister's office, told reporters on Monday that a fact-finding committee calculated the scheme had caused economic losses of 286.6 billion baht (S$11 billion).

Yingluck alleges bias in the way the case was handled.

She was thrown out of office by a Constitutional Court ruling in May 2014 just before the military staged a coup against the remnants of her caretaker government. Last year, she was retroactively impeached by a military-appointed legislature, which banned her from politics for five years. Yet she remains a popular figure among Puea Thai supporters, and has been keeping up her public profile through provincial visits.

Thailand's agricultural sector has been flagging in the past two years under military rule, as the withdrawal of generous farm supports has coincided with weak commodity prices and unusually dry weather that left the lands parched. Farmers who loaded up on debt during good times have been forced to sell their land - or take up even more expensive debt to keep afloat.

Weighed down by high household debt - it remained at over 80 per cent of the gross domestic product last year - the Thai economy grew at a lower-than-expected 2.8 per cent.

On Thursday, the commerce ministry announced that it was prepared to spend 3.9 billion baht to buy rice during the harvest season starting in November to prop up prices, reported The Nation.

Yingluck urged Thais to turn out in force for tomorrow's nationwide referendum.

Some 50 million voters will decide on the fate of a draft Constitution drawn up under the watch of the junta. This draft charter, which will be Thailand's 20th if it is adopted, commits future governments to a "national strategy plan" and also gives the junta broad oversight through a strengthened senate during a transitory period.

"I would like to invite all Thais to exercise their rights… We should do our best so that we won't regret the result," she told reporters.