Amid heightened tensions, former Thai premier Yingluck Shinawatra has urged her supporters to stay away from court today as the verdict on her criminal negligence trial - which could land her in jail - is read.

"Even though we know that every one of you will come without any bad intention, it may be perceived in other ways," she wrote in a Facebook post yesterday. "I don't want your participation to be used to cause violence or result in intervention by others."

Police officers will swarm the courthouse just outside Bangkok today in anticipation of the thousands of Yingluck supporters expected to turn up. Meanwhile, large mysterious signs urging "no disunity, no fracture" appeared in dozens of locations in Bangkok yesterday, according to a Reuters report.

Yingluck and former commerce minister Boonsong Teriyapirom both face possible imprisonment in separate trials related to the ousted government's multibillion-dollar subsidy for rice farmers. Yingluck, who was ousted from her position as prime minister in 2014 shortly before the military staged a coup, is accused of criminal negligence for failing to rein in the policy, under which the state bought rice from farmers at inflated prices. If convicted, she could be jailed for up to 10 years.

Boonsong is accused of corruption over a 2013 deal to sell stockpiled rice to the Chinese government, a deal later declared bogus by Thailand's national anti-graft body. If convicted , he could be jailed for life.

Guilty verdicts would appease the section of the polarised country which took to the streets three years ago to topple the Puea Thai party-led government. But emotions could run high in the other camp, under tight security surveillance since the coup.

"If (Yingluck) is jailed, it would start a fire in Thai politics," Chiang Mai University historian Attachak Sattayanurak told The Straits Times.

Thailand's first female prime minister is already banned from politics until 2020, after being retroactively impeached by a military-appointed assembly in 2015.

But the 50-year-old former business executive is still arguably Puea Thai's most recognisable personality alongside her brother Thaksin Shinawatra. A former premier ousted by a coup in 2006, he lives in self-imposed exile to evade a graft-related jail sentence.

The rice-pledging scheme was a key plank in Puea Thai's 2011 election campaign. While it raised farmers' incomes, it also bloated government stockpiles, distorted prices and toppled the kingdom from its position as the world's top rice exporter.

While playing down the possibility of major unrest, the authorities have taken no chances. In the last few weeks, police and soldiers visited Puea Thai supporters to dissuade them from turning up outside the courthouse today.

"They came to my house and my office," said Dr Pongsak Pusitsakul, a surgeon and political activist in Ratchaburi province. "They say 'we are concerned about you going to Bangkok on Aug 25. Will you go?'"

Van drivers have also been warned against transporting Yingluck's supporters to Bangkok, said activists outside the capital.

But this has not deterred people like Dr Pongsak.

"If she is sent to prison, we will move from outside the courthouse to the jail," he said.