The May Day march on Tuesday was limited to a riverside space near the city’s landmark site of Wat Phnom and with a large security force presence.
Bun Chhay was arrested in August last year over a 2007 drugs case and another drugs bust in 2012 and charged with drug trafficking offenses.
Rhona Smith said the government must immediately release the detained opposition leaders and lift a ban on the opposition taking part in the July 29 general election.
Aun Pheap is facing charges related to reporting before June 2017 local elections, as is his Canadian reporting partner.
While the political relationship between Cambodia and China continues to grow in strength, signs of tensions have begun to show as fears over criminal activity circulate.
The CNRP was dissolved and its lawmakers banned from politics in November after the Supreme Court ruled that it had tried to overthrow the government.
Though born in Hawaii, raised in Indonesia, and educated in New York City, former President Barack Obama has adopted city of Chicago as his hometown. VOA Khmer's Sreng Leakhena narrates.
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.