Killing comes as opposition accuses Prime Minister Hun Sen's government of launching a fresh crackdown against them.
A prominent Cambodian political analyst known for his criticism of the government has been shot dead at a cafe in the capital Phnom Penh, police said.
The killing of Kem Ley on Sunday comes at a time of heightened political tensions between Prime Minister Hun Sen and the opposition, who accuse him of pursuing a fresh crackdown against them.
Police said Kem Ley, a popular commentator and grassroots campaigner, was gunned down as he drank coffee at a shop attached to a petrol station.
"He was shot dead at a mart just before 9am," Kirt Chantharith, national police spokesman, told the AFP news agency.
A suspect was arrested nearby and confessed to killing the analyst over an unpaid debt, Kirt Chantharith said.
"But we don't believe him yet. We are working on this case," he said.
Kem Ley 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.
But the bulk of his criticism was aimed at Hun Sen's ruling party.
As news of Kem Ley's murder spread on social media, hundreds of Cambodians descended upon the Caltex station.
Ky Sour, a local businessman, said the crowd had gathered because they were "heartbroken" over Kem Ley's death, but also because they did not "trust the police.
"They suspect the police will do something to hide the evidence [if they don't watch]," Ky Sour said.
"This is the same pattern, so everyone knows who is behind it. People who have no power cannot do like that. This is the face of dictatorship."
A factory worker at the scene, who asked not to be named, citing safety concerns, said: "Sometimes we cannot talk about what we think about the government.
"We close our mouths and discuss with our family or close friends only."
Cambodia has a history of rights and labour advocates being murdered with killers rarely brought to justice.
Kem Ley's murder is likely to exacerbate tensions in the Southeast Asian nation, which has been dominated by Hun Sen for the past 31 years in a reign marked by accusations of corruption, electoral fraud and rights abuses.
Scores of government critics and rights workers have been arrested in recent months, while others have been tied up in ongoing legal cases.
The prime minister, a former army commander who defected from the Khmer Rouge, has held power alongside a small but powerful group of political allies who have become enormously wealthy.
A report published by Global Witness last week detailed how Hun Sen's family had amassed a multimillion-dollar business empire spanning the country's most lucrative sectors during his rule.
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