By Prak Chan Thul
PHNOM PENH (Reuters) - Hundreds of thousands of supporters of both Cambodia's ruling party and the opposition held rallies in Phnom Penh on Friday ahead of local elections on Sunday seen as a test of support for veteran strongman Hun Sen ahead of a 2018 general election.
Prime Minister Hun Sen, a former Khmer Rouge soldier, has warned of possible violence if his ruling party does not keep control of the Southeast Asian country, which was devastated by genocide in the 1970s.
Rivals, and human rights groups, accuse Hun Sen of using court cases, arrests, intimidation and other measures to keep a more than three-decade hold on power, making him one of Asia's longest serving leaders.
In a sign of election pressure, Hun Sen broke with his longstanding tradition of avoiding direct campaigning to join an estimated 200,000 supporters of his Cambodian People's Party (CPP).
"The opposition has done nothing besides telling foreign donors to cut aid, investors not to invest and the EU not to import Cambodia garments," Hun Sen said in an hour long speech to the crowd gathered in the tropical heat.
"This is the choice between peace and security for development, and chaotic destruction," Hun Sen said.
During the Khmer Rouge regime of the 1970s some 1.8 million people died from starvation, torture, exhaustion or disease in labor camps or were bludgeoned to death during mass executions
Since Hun Sen came to power, Cambodia has emerged from decades of conflict to clock annual growth rates above 7 percent, a record he highlighted.
But the opposition performed unexpectedly well in the 2013 general election after uniting behind a single party.
The opposition Cambodia National Rescue Party (CNRP) is expected to again improve its showing in Sunday's communal elections, given that divided opposition parties secured only three percent of communes at the last such ballot.
At a CNRP rally on Friday, party member Yim Sovann told supporters not to be afraid and promised a landslide win in 2018.
"We all need to be united as one and win," CNRP lawmaker Yim Sovann said, promising that the opposition would do three times as much for Cambodia as the Hun Sen's party, which it accuses of corruption.
Cambodian political analyst Cham Bunthet said that if the CNRP won over 31 percent of the votes on Sunday, it would be a blow to the ruling party, but he said the opposition's chances could be weakened by low voter turnout.
Sebastian Strangio, author of the book, "Hun Sen's Cambodia" said the election would not affect the ruling party's control of the national government.
"But a heavy loss could give the opposition momentum that it can ride towards the real event: next year's national election," he said.
(Editing by Matthew Tostevin and Michael Perry)