PHNOM PENH (AFP) - Rights groups have lambasted Cambodian strongman Hun Sen for using "violence, repression and corruption" to keep his grip on power as he marked three decades as prime minister on Wednesday.

The former Khmer Rouge cadre became the world's youngest premier when he took office on January 14, 1985 at the age of 32.

But his administration has been mired in graft and he is frequently criticised for ignoring human rights and stamping out dissent.

In a report released Tuesday, Human Rights Watch accused the 62-year-old of ruling through violence, control over the security forces 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 fell 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 and eventually returning with Vietnamese troops to oust the regime, later climbing to the top of the Hanoi-installed government in Cambodia.

As his country 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, adding that "dissent is aggressively stifled".

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.