TAKEO • Thousands gathered yesterday at the grave of a prominent Cambodian critic who was gunned down a year ago in a murder that sparked widespread anger and scepticism over the alleged killer's motives.

Popular and charismatic political analyst Kem Ley, 46, was shot twice in the head as he sipped coffee in Phnom Penh - a brazen assassination that sent shock waves through the country's already beleaguered activist community.

Former soldier Oeuth Ang, who was unemployed, admitted carrying out the killing and was given a life sentence in March after a brief trial. His declared motive, that the murder was revenge for an unpaid US$3,000 (S$4,000) debt, was met with disbelief. He was not cross-examined in court because he had effectively admitted guilt.

"I don't know whether they made up that debt story but I don't believe it at all," Mr Kem Ley's mother Phok Se, 77, told AFP as well-wishers gathered at the family home in Takeo province.

"There has been no justice for us so far," she added, echoing the sentiments of many at a ceremony which saw Buddhist monks chant prayers as devotees made offerings around the grave.

In Phnom Penh, police stopped mourners from placing flowers at the petrol station cafe where Mr Kem Ley was gunned down.

In a joint statement to mark the anniversary, more than 100 local and foreign organisations called on the government to reopen the case after a "flawed trial".

"There has been no transparency in the murder investigation and there are still many unanswered questions in this case," the statement read, adding that there was "compelling evidence" showing Oeuth Ang had accomplices.

Many friends and supporters find it hard to believe Oeuth Ang, who rarely held down a job, could afford to lend US$3,000 - more than twice the average annual salary in Cambodia.

Cambodia has been ruled by strongman premier Hun Sen for 32-years and has a dark history of usually unsolved activist killings. In the 1990s and early 2000s, such assassinations were common, but they had become rarer in recent years.

Spooked that the bad days may have returned, tens of thousands turned out for Mr Kem Ley's funeral, rattling the government in some instances.

Mr Kem Ley criticised Cambodian politicians of all stripes, but was particularly scathing about the corruption that blights the country. Shortly before his murder, he gave a radio interview about an investigative report that detailed some of the millions of dollars amassed by Mr Hun Sen's family.

The government has strongly denied any responsibility in his killing. Mr Hun Sen faces crunch national polls next year.

AGENCE FRANCE-PRESSE