Reuters and the Phnom Penh Post staff said they were told the 30-seat courtroom was full while a few others were allowed to enter.
Kem Sokha (pictured) is accused of plotting to overthrow strongman Prime Minister Hun Sen, who has been in power since 1985.
The charges against Kem Sokha have been dismissed as politically motivated by human rights groups.
Kem Sokha faces up to 30 years behind bars although some observers say he could be convicted and then pardoned to protect a trade agreement with the European Union.
An EU decision on preferential trading rights is expected in mid-February as Brussels has threatened to withdraw over human rights abuses.
The EU said the Everything But Arms (EBA) was put at risk because of Kem Sokha’s arrest and the opposition crackdown.
Observers say the looming EBA decision will have a significant influence on the trial.
All visitors to the 30-seat court were required to register in advance for the trial that observers believe could last up to three months.
Kem Sokha and his former political rival Sam Rainsy founded the Cambodia National Rescue Party (CNRP) in 2012.
At the 2013 general election, the CNRP came within seven seats of defeating Hun Sen’s Cambodian People’s Party.
A former Khmer Rouge cadre, Hun Sen, 67, fled Cambodia in 1977 and returned with the Vietnamese military during that country’s war against the savage regime in 1979. He was first appointed foreign minister and was named prime minister in the Vietnamese-supported government in 1985.
In September 2017, Kem Sokha’s home was raided and he was accused of plotting a US-backed insurrection.
The “incriminating” evidence was footage of Kem Sokha telling an event in Australia that he received US political advice on how to build grass-roots support.
Hun Sen outlawed the CNRP and won all 125 seats in the 2018 general election.
Sochua, the vice president of the CNRP, said the trial was a farce.
“They will find him guilty as charged then proceed with a royal pardon,” she said.
Pardons have already been granted by King Norodom Sihamoni to other opposition chiefs, including Sam Rainsy.
Phil Robertson of Human Rights Watch said: “Kem Sokha will be the victim of a staged trial on completely bogus treason charges.
“The reality is Kem Sokha did nothing he should have been charged for and this entire pre-trial prison detention, house arrest and now the trial has been a massive violation of his human rights,” he added.
Nicholas Bequelin of Amnesty International said: “The authorities are clearly trying to limit scrutiny of this politically motivated trial.
“It is essential that human rights monitors and journalists are given unhindered access to the trial.”
The NGO said Cambodia’s authorities “have not presented a shred of credible evidence to support a charge of treason”.
Kem Sokha in November. Picture credit: Wikimedia