The group was sheltering from a downpour in a mountainous area of Koh Kong province's Thmar Baing district on Thursday, police chief for minor crimes Lay Meng Laing told AFP, adding that three victims were from the same family.
They had just finished breakfast together when he left to meet some students for a chat over coffee. She received a call soon after, informing her that he had been gunned down in broad daylight.
Tens of thousands of people attended the event organised by Hun Sen's ruling Cambodian People's Party, which has dominated the country since it was installed by the Vietnamese forces which toppled Khmer Rouge leader Pol Pot on January 7, 1979.
Hun Sen, a firebrand former Khmer Rouge fighter who has held office for 32 years, has already promised a guilty verdict will be delivered on Thursday -- a ruling most agree is inevitable in a justice system firmly under the premier's thumb.
Opponents of Hun Sen, as well as NGOs and the critical press, have increasingly been smothered by court cases and threats before a crucial general election next year.
The order comes a day after the strongman premier threatened the Cambodia Daily, one of the country's few remaining critical newspapers, with closure over an alleged unpaid tax bill of $6.3 million, calling them "thieves".
Nhek Bun Chhay, a former army commander-in-chief and deputy prime minister, was arrested weeks after rumours surfaced that he had discussed a political tie-up with Hun Sen's main opposition.
More than seven million people voted on June 4, testing the political temperature of a country rife with tension between Prime Minister Hun Sen and an embattled opposition determined to end his more than three-decade rule.
Vote is widely seen as a test ahead of next year’s national elections
The two Phnom Penh residents -- a man and a woman -- were arrested on Wednesday for at least ten cases of "kidney trafficking" over the past year, said Keo Thea, the city's chief of anti-human trafficking police.
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Hun Sen has long maintained that violence would break out if his party were ousted in elections.
More than 500 felons have been sent to Cambodia through a repatriation deal, though many were raised in the US and arrive in the country having never visited and unable to speak the language.
Egyptian, El Salvadoran human rights defenders also shortlisted
The verdict is the latest legal case to hamper an opposition movement struggling to break Prime Minister Hun Sen's 32-year grip on power.
Oeuth Ang says killing because of unpaid debt
The damning assessment comes as Cambodia plans to hold nationwide polls next year in what some have warned could be the country's last chance of seeing genuine democracy take root.
NGOs say violence against women happens often, discrimination present in all sectors of industry
Sam Rainsy, who had been at the forefront of the kingdom's opposition movement for two decades, quit as leader of the Cambodia National Rescue Party (CNRP) in a letter posted on social media last month.
Ven Sopheap, a resident of eastern Prey Veng province, admitted during his trial last week to posting threats against Hun Sen -- the authoritarian premier who has ruled Cambodia for more than three decades.