Phnom Penh (AFP) - Cambodia's strongman premier on Wednesday told his critics and political opponents they faced elimination and should "prepare coffins", an escalation in rhetoric even for a man known for bellicose speeches.

Prime Minister Hun Sen has ruled the impoverished nation for 32 years, making him one of the world's longest serving leaders.

He portrays himself as a leader who has brought growth and stability while critics say corruption, inequality and right abuses have become entrenched during his years in office.

The 64-year-old delivered a string of increasingly shrill speeches in the run up to local elections earlier this month, which saw the main opposition party make significant gains.

But on Wednesday he delivered one of his most vitriolic speeches yet, warning of civil war if his party is toppled in next year's general election.

Flanked by top military brass and senior government officials, Hun Sen said troops were ready to "crack down on all movements that would topple (the government) and damage the nation."

"Your tongues are the cause of war," he said of his critics. "If you guys keep insulting, threatening to kill, you guys should prepare coffins. I'm warning you," he added.

Hun Sen made the remarks dressed in military uniform, clothing he rarely adopts, during a visit to the Vietnamese border.

The speech coincided with the 40th anniversary of his decision to defect from the Khmer Rouge army and cross into Vietnam, which later invaded Cambodia and toppled the genocidal regime that killed nearly a quarter of Cambodia's population.

Hun Sen's speech made specific threats to kill opponents.

"I have said that in order to ensure peace and protect the lives of millions of people, if it is necessary to eliminate 100 or 200 people, they must be eliminated," he said.

Cambodia's embattled opposition fared well in local elections on June 4 according to early results, suggesting it has managed to grow its appeal despite an extended government crackdown on its network and supporters.

The vote was closely watched for signs of Hun Sen's vulnerability and full results will be published later this month.