Tens of thousands of Cambodians poured on to the streets of Phnom Penh for the funeral procession of a prominent political analyst murdered in a brazen daylight shooting that shocked the Southeast Asian nation.

Kem Ley, a popular pro-democracy voice and grassroots rights activist, was shot dead earlier this month while drinking coffee outside a petrol station in the capital, raising suspicion of a political conspiracy.

On Sunday, a massive crowd of mourners trailed for kilometres behind Buddhist monks and a motorcade carrying the 46-year-old's body in a transparent casket for a journey to his hometown in Takeo province.

Thousands of others lined the streets to watch the procession, which marked the end of a two-week mourning period that saw scores of people from across the country flock to the Phnom Penh temple where his body had lain.

"He was a mirror of society, a hero. His murder is a huge loss to democracy," 39-year-old Hul Chan told AFP.

Many of them wore white shirts printed with Ley's face and the words: "Wipe your tears, continue your journey".

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A former soldier charged with the murder claimed he shot Ley over an outstanding debt.

But suspicions of an assassination continue to mount as critics have come under increasing pressure from the government of Hun Sen as he seeks to extend his 31-year grip on power.

"[Ley] was a mirror of society, a hero. His murder is a huge loss to democracy," Chan told AFP while he was walking alongside other mourners.

Ley, a popular radio commentator, was a regular critic of Hun Sen as well as the political opposition. He called for a new era of clean politics in Cambodia - one of the most corrupt countries in the world.

He was also a major advocate for land and labour rights, travelling across the impoverished country to speak directly with villagers.

Am Sam Ath, from the Cambodian rights group Licadho, said Sunday's outpouring of grief reflected the people's growing disillusionment with a government accused of rampant corruption and rights abuses.

"Kem Ley was a straight-talking person who strongly criticised social injustices," he told AFP.

"The people are standing up against injustice."

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Anti-riot police were on guard outside government buildings along the route.

Hun Sen has promised a thorough investigation into the killing but has urged people not to turn the case into a "political act".

Shortly before Ley's death, the activist gave a lengthy radio interview welcoming a report which detailed the riches amassed by Hun Sen and his family during his three decades in power.

He was critical of both the government and opposition parties, advocating for a new era of clean politics in Cambodia, which is expected to hold a general election in 2018. 

Scores of government critics and rights workers in Cambodia have been arrested or silenced in recent months, while others have been tied up in ongoing legal cases.

Source: Agencies