Security is high in Thailand ahead of a court ruling that could see the two main political parties disbanded and their leaders banned from politics.
Thousands of police are guarding the court building
Thousands of police have been stationed around the capital Bangkok to await the Constitutional Court verdict.
The Thai Rak Thai (Thais Love Thais) and the Democrat Party are accused of fraud during the April 2006 election.
Former PM Thaksin Shinawatra, who headed Thai Rak Thai, was ousted in a bloodless military coup last September.
The Thai King warned the judges last week that the future stability of the country rested on the fairness of their decision.
More than 2,000 police had been stationed around the courthouse on Wednesday morning and checkpoints set up into and out of Bangkok, police said.
Some 10,000 security officers were on standby and an evacuation plan ready for the judges if their decision sparked street protests and violence, military officials said.
The two parties are accused of violating election law during last year's political crisis.
A series of street protests led to Thai Rak Thai calling a snap election, which the Democrat Party boycotted.
In the aftermath of the poll, both parties were accused of trying to manipulate results and violating election laws. The election was later annulled, and after weeks of turmoil the military staged its coup.
Mr Thaksin, who is now living abroad, and more than 100 other prominent politicians from both parties could be barred from office for five years if found guilty.
The two parties could also be dissolved, although they would be able to register under different names for future elections.
King Bhumibol Adulyadej - who has no formal powers but is highly revered in Thailand - gave a rare televised address on the issue last week.
He said the decision was "highly important", and told the judges: "You have the responsibility to prevent the country from collapsing."
But whatever the court verdict, it is unlikely to clear the air in what is already a damaged democracy, the BBC's Asia correspondent Andrew Harding says.
The military leadership has promised new elections and a constitution by the end of this year.
But frustration with the interim government is growing and the economy is struggling.