By Prak Chan Thul

PHNOM PENH (Reuters) -Cambodia's Hun Sen, one of the world's longest-ruling leaders, has indicated his eldest son and anointed successor Hun Manet might take over as soon as next month in a surprise revelation just days out from a one-sided election.

It had been widely expected that the U.S. and British-educated Hun Manet, 45, would take over from his father, who has ruled Cambodia for nearly four decades, but no timeframe had previously been given.

"In three or four weeks, Hun Manet can become the prime minister. It depends on whether Hun Manet will be able to do it or not," Hun Sen said in an interview with China's Phoenix TV aired on Thursday.

The self-styled strongman and former Khmer Rouge guerrilla Hun Sen turns 71 next month. Hun Manet, one of Cambodia's top military generals and former chief of his father's bodyguard unit, makes his debut as a candidate in the election on Sunday.

The Cambodian People's Party (CPP) is running virtually unopposed, after its only rival of significance was disqualified on a was disqualified on a technicality over its registration paperwork.

Human rights groups have dismissed the election as a sham and accuse Hun Sen of controlling the media, threatening critics, and systematically dismantling his opposition using trumped-up criminal charges.

The government denies persecuting its opponents.

SMOOTH TRANSITION

Many analysts believe the crackdown on the opposition was designed to ensure the election was effectively a one-horse race to enable a smooth transition of power for his son.

Hun Manet attended the West Point military academy in the United States and has a master degree from New York University and a doctorate from the University of Bristol, both in economics.

Asked on Friday about the succession, a CPP spokesperson Sok Eysan said CPP had two prime ministerial candidates - Hun Sen and Hun Manet - and there was "nothing more to confirm" than what the leader said in the television interview.

Manet has kept a low profile outside of Cambodia during his rise through the military ranks and since being gradually introduced to domestic politics.

It is unclear what kind of leader he might be, given his years living both in Western democracies and back home under his father's autocratic rule.

Hun Manet appeared at a rally in the capital Phnom Penh on Friday on the last day of campaigning and expressed confidence in a landslide victory for the CPP, which he said was the only party capable of governing and had the full backing of the public.

"They have committed to voting for the Cambodian People's Party to ensure a bright and prosperous future of the nation for the next generation," a smiling Hun Manet told a cheering crowd.

He spoke of how the CPP had maintained peace and stability after decades of war and said an unspecified group of evil "extremists" was trying to "destroy this election", in rhetoric similar to that of his father.

(Additional reporting by Ella Cao; Writing by Martin Petty; Editing by Andrew Heavens and Angus MacSwan)