Hong Sok Hour, a senator from the opposition Cambodia National Rescue Party, appeared in court in an orange prison uniform and opened the day's proceedings with a plea for bail.
He told the court he has high blood pressure and other medical conditions that require medication he was not getting in prison.
"I will not run away if you release me on bail," said the senator, who has been held in pre-trial detention since his August 15 arrest.
Human Rights Watch and other international rights groups have called on authorities to drop the case against Hong Sok Hour, saying he had been wrongfully charged and that prosecuting him marked the government's latest crackdown on the political opposition.
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Hong Sok Hour was arrested after Prime Minister Hun Sen accused him of treason for online comments that criticised a 36-year-old border agreement with Vietnam.
He was indicted on three charges including falsifying public documents, using fake documents and inciting chaos. The charges carry penalties of up to 10 years in prison, up to 5 years and up to 2 years, respectively.
The arrest was the latest in a string of attacks on Hun Sen's political opponents. In recent months, Hun Sen has used his public speeches to deliver what amounts to arrest orders, which are generally carried out quickly.
The prime minister accused Hong Sok Hour of posting material about the 1979 border agreement that "amounts to treason." Hun Sen was foreign minister at that time in a government installed by a Vietnamese occupation force that invaded Cambodia to oust the murderous Khmer Rouge regime.
Hun Sen has been in power for almost three decades, and while Cambodia is formally democratic, his government is authoritarian and known for intimidating opponents.