Phnom Penh - Tens of thousands of Cambodians on Sunday joined the funeral procession of a prominent government critic whose murder in broad daylight raised suspicions of a hit job.Kem Ley, a popular pro-democracy voice and grassroots rights activist, was shot dead two Sundays ago while drinking coffee outside a petrol station in the capital.The murder sent ripples of fear across the country. Critics have come under increasing pressure from the government of strongman Hun Sen as he seeks to extend his 31-year grip on power.A former soldier charged with the murder claimed he shot Kem Ley over an outstanding debt.But suspicions of an assassination continue to mount in a country where the rule of law is threadbare and critics of the elite are easily silenced.On Sunday a massive crowd of mourners, many carrying portraits of Kem Ley, trailed for kilometres behind Buddhist monks and a motorcade carrying the 46-year-old's body in a transparent casket.Thousands of others lined the streets to watch the procession, which marked the end of a two-week mourning period that saw 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 while he was walking alongside other mourners.Many of them wore white shirts printed with Kem Ley's face and the words: "Wipe your tears, continue your journey."'Straight-talker' Sunday's procession, the largest public gathering in the country in recent years, will see the activist's corpse returned to his home village some 70 kilometres south of the capital for burial.Kem Ley was a regular critic of Hun Sen as well as of the political opposition. He called for a new era of clean politics in Cambodia, one of the world's most graft-ridden countries.The frequent radio commentator was also a major advocate for land and labour rights, travelling across the impoverished country to speak directly to 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."Shortly before his death Kem Ley 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.United Nations rights experts expressed concern over the circumstances of Kem Ley's killing. In a statement they said it "exemplifies an alarming negative trend in Cambodia whereby political activists and human rights defenders are facing increasing restrictions".Hun Sen has ordered a thorough investigation into the murder and urged people not to turn the case into a "political act".