Cambodia is placed 142nd out of 180 countries in this year’s World Press Freedom Index.
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.
The US and EU have threatened Cambodia with deepening economic sanctions following the ruling party’s decision to dissolve the opposition Cambodia National Rescue Party.
Aun Pheap is facing charges related to reporting before June 2017 local elections, as is his Canadian reporting partner.
Many demonstrators call for better working conditions, higher pay.
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.
Singapore's prime minister warns against 'protectionism and anti-globalization sentiments'.