Cambodian strongman Hun Sen marked three decades as premier on Wednesday hailing his role in rebuilding the war-torn nation, as rights groups lambasted him for using "violence, repression and corruption" to cling to power.

The former Khmer Rouge cadre became the world's youngest prime minister when he took office on January 14, 1985 at the age of 32, his humble back story and sharp wit aiding his reputation for being in touch with ordinary Cambodians.

But his administration has been widely criticised for graft, while Hun Sen stands accused of ignoring human rights abuses, stamping out dissent and rigging elections.

In a report released Tuesday Human Rights Watch accused the 62-year-old of ruling through violence, control of the security apparatus and manipulated elections to become the world's sixth-longest serving political leader.

"For three decades, Hun Sen has repeatedly used political violence, repression, and corruption to remain in power," said Brad Adams, HRW Asia director, in a statement.

"Cambodia urgently needs reforms so that its people can finally exercise their basic human rights without fear of arrest, torture, and execution. The role of international donors is crucial in making this happen," he said.

When Cambodia collapsed into civil war in 1970, Hun Sen became a foot soldier for what later emerged as the Khmer Rouge -- the genocidal regime that killed up to two million people.

He rose to the rank of deputy regional commander before defecting to Vietnam, eventually returning with Vietnamese troops to oust the regime in 1979 and later climbing to the top of the Hanoi-installed government in Cambodia.

"I thank people who say I am bad. I thank people who say I am good. I thank all of them," Hun Sen said at ceremony to mark construction work on a bridge over the Mekong River, in Neak Loeung, 60 kilometres (40 miles) southeast of Phnom Penh.

"Without Hun Sen's hands, there would have no Paris peace agreement," he said, referring to the 1991 deal giving the UN authority to supervise a ceasefire and democratic elections after years of bloody civil war.

"If Hun Sen did not enter the tigers' hole, could we arrest the tigers?" added the premier, heralding his role in eradicating Khmer Rouge strongholds. "Indeed I have made some mistakes. But please balance the right and wrong ones."

Stifling of dissent

As Cambodia emerged from conflict, Hun Sen abandoned the communist dogma of his Vietnamese patrons, embracing the free market and seeking out alliances with more powerful nations.

Yet while Cambodia now enjoys relative stability, the authoritarian premier "has nurtured a system in which political power is based on alliance to the ruling party", said Sopheap Chak, executive director of the Cambodian Center for Human Rights in Phnom Penh.

"The government has failed to establish the rule of law or combat impunity... serious human rights violations regularly occur," she said.

Rising discontent over forced evictions and growing inequality in Cambodia, one of the world's poorest countries, have led to a surge in support for the opposition Cambodia National Rescue Party (CNRP).

The CNRP boycotted parliament for nearly a year after accusing Hun Sen of rigging the 2013 general election that returned his Cambodian People's Party to power. They only took up their seats in July in return for a promise of electoral and parliamentary reforms.

"Any country, for the sake of democracy and progress, has to renew its leadership," CNRP leader Sam Rainsy told AFP, adding that even communist countries like China and Vietnam change their leaders every few years.

But on Wednesday the wily Hun Sen said he would stay in power until at least 2018, when the next general election is scheduled, and that any longer would depend on voters.

He has previously vowed to rule until he is 74.