Aun Pheap, 54, now in the United States, was charged in August last year along with a colleague, Zsombor Peter, a Canadian, after an interview with former opposition members.
The Cambodian Senate has long been seen as a rubber stamp body with the balance of power falling with the ruling party and its cronies.
More than 1,000 Chinese nationals have been arrested and deported from Cambodia since 2012 relating to online scams being run out of Cambodia.
The blossoming friendship between China and the Cambodian government has become so close that a Chinese majority-owned, Khmer Language television station has built studios directly inside the Ministry of Interior in Phnom Penh.
David Chandler saw positive changes in terms of liberalization and democratization when he was concluding the current version of the book, which was last revised in 2007, but in the past decade Cambodia has taken backward steps, he said.
Sam Rainsy added that Japan had strong leverage over Cambodia due to its considerable development assistance and investment.
Hun Sen ordered those involved in the plot arrested and for the police to step up security in the capital.
The World Khmer Conference participants also demanded the immediate release of Kem Sokha and other political prisoners and called for free and fair elections.
A ruling Cambodian People’s Party spokesman said Sam Rainsy’s appeal to voters was “in vain”, adding that turnout was expected at between 60 and 70 percent.
Prime Minister Hun Sen on Monday blessed the Cambodian people during a Khmer New Year ceremony, declaring his victory over the opposition party.
Long Sokunthy, 42, represents some 300 people who are locked in a long-running land dispute with a local tycoon.
Foreign Minister Taro Kono said that Tokyo would help Cambodia with free and fair elections, according to a spokesman for his office, Norio Maruyama, who confirmed that Japan would supply ballot boxes for the July polls.