The mysterious murder of a top Cambodian activist and a spike in violence at land and labour disputes mean the country's culture of impunity is worsening, campaigners and opposition politicians say.

Chhut Vuthy was gunned down on April 26 by a military policeman as he tried to expose illegal logging and his killer, apparently overcome by remorse, then shot himself twice in the chest with an AK-47, according to officials.

Human rights groups say the explanation smacks of a cover-up.

"For the past six months what we are seeing is more and more cases of extreme violence in land and labour disputes where arms are being used," said Naly Pilorge, director of local human rights group Licadho.

Thousands of Cambodians protested for better pay and conditions in the country's key garment industry in February, when a local governor allegedly fired into the crowd, wounding three women who work for a supplier to German sports giant Puma.

Bavet city governor Chhuk Bundith was merely charged with the low-level crime of causing "unintentional injuries" in late April, and has yet to be arrested.

The incidents are examples of what critics say is an increasingly brutal approach to rights campaigners.

Chhut Vuthy had been instrumental in helping the media document deforestation and logging, and two women reporters from a local newspaper were with him when he was shot.

In a written account, the journalists said they did not see who pulled the trigger, but stated they feared for their safety in the following moments when they overheard a soldier saying: "Just kill them both."

"This kind of violence is to be expected when rule of law completely breaks down and impunity for officials is an everyday occurrence," said Phil Robertson, deputy Asia director for Human Rights Watch.

Not including Chhut Vuthy's death, Licadho has documented five shootings in land disputes since November, with 19 people wounded, and said in each case military police acting as security guards for private firms had opened fire.

But Cambodian government spokesman Ek Tha rejected suggestions that campaigners or protesters were at risk.

"The royal government of Cambodia has no policy of violence, it respects the rule of law," he told AFP, blaming examples of excessive force by some military police officers and policemen on "individuals' activities".

Conflicts over land are arguably Cambodia's biggest problem, with ownership a murky issue after many legal documents such as land titles were destroyed by the hardline communist Khmer Rouge in the late 1970s.

Cambodia's government granted some 800,000 hectares (two million acres) in land concessions to well-connected private firms in 2011, according to local rights groups. The figure represents roughly five percent of the country's entire area.

A further 300,000 hectares have been leased this year already, they say, accusing authorities of a land grab, and the number of protests is rising. "Desperate people are taking matters into their own hands," Robertson told AFP.

In one of the worst confrontations to date, during the eviction of some 300 families from Phnom Penh's Borei Keila neighbourhood, locals threw petrol bombs at police, who responded with tear gas and rubber bullets.

Long-time Cambodia watchers said the January riot was the first time they had seen residents using Molotov cocktails -- perhaps an indication that protesters are themselves growing bolder.

Leading opposition politician Mu Sochua said the violence was a consequence of a "lack of democracy" under strongman Prime Minister Hun Sen, who has been in power since 1985.

Not since the 2004 daylight murder of union leader Chea Vichea has Cambodia lost an activist as influential as Chhut Vuthy, she added, accusing donor countries of "making no noise" in support of ordinary Cambodians' rights.

"I want to be optimistic, I want to see hope but I'm afraid there is no more Chea Vichea, and there is no more Chhut Vuthy," she said.

"They cannot be replaced. That is the aim of those who ordered the killings."