It was a shocking sight for many Cambodians — their strongman Prime Minister standing on the back of a campaign truck, waving and sweating in the monsoon humidity.

Typically, Hun Sen has stayed away from pre-poll campaigning, comfortable in the knowledge that his Cambodian People's Party (CPP) will dominate.

He has, after all, ruled for 32 years.

"It was strange, he never appeared in past campaigns," said Srey Pich, a 26-year old bank worker.

"I guess he wants to test his popularity and try to attract votes," she said, relaxing in a park in front of Hun Sen's gaudily-lit Phnom Penh mansion.

Even though today's election will choose commune-level representatives who mostly deal with births, deaths and marriages, it's being fought out as if the stakes are much higher.

"Hun Sen and senior members of his party have expressed publicly [and] repeatedly that they see this as a battle for the future of their country and indeed for the survival of the country," said Sebastian Strangio, author of Hun Sen's Cambodia.

There have been wild words on both sides.

The Opposition has promised to give US$500,000 ($670,000) to each of the 1,646 communes, while Hun Sen has threatened a return to civil war if he loses.

Cambodia's Prime minister Hun Sen (centre) has been in power for 32 years. ( Supplied: The Phnom Penh Post (file photo) )

It's a threat he's made before, but this time Hun Sen has gone further, specifying how many Cambodian lives are expendable in the pursuit of power.

"To ensure peace and to continue the development, the only option is that the CPP must win elections at all stages," said Hun Sen this week, according to the Cambodia Daily newspaper.

"To ensure the lives of millions of people, we are willing to eliminate 100 or 200 people because we have seen bitter past experience," said the Prime Minister, who defected from the Khmer Rouge and was placed into power by the Vietnamese.

It's no empty threat.

In 1997, forces loyal to Hun Sen killed dozens of political opponents in street battles that turned Phnom Penh into a war zone for two days.

A year later, the CPP won the general election by a comfortable margin.

High hopes after shock opposition gains

This is the first major vote in Cambodia since the shock result of 2013, when the Opposition surged to within 13 seats of victory.

The commune election will test the political mood before national elections next year and potentially offer a morale boost.

While there are 12 parties registered, there are only two real players — the ruling CPP and the opposition Cambodian National Rescue Party (CNRP).

Opposition leader Kem Sokha hopes to get 60 per cent of the vote, which is a remarkable ambition considering the CPP secured 97 per cent of commune chief positions in 2012.

Since then, the two main opposition factions have united, and Cambodians have had five more years of graft and cronyism from Hun Sen's Government.

Kem Sokha, president of the opposing CNRP, hopes to get 60 per cent of the vote. ( AP: Heng Sinith )

"A lot of the problems that existed prior to 2013 — land grabbing, deforestation, the wide spectrum of day-to-day corruption that ordinary people have to put us with — all of these things have continued," said Mr Strangio.

Pre-election polls are outlawed, but anecdotally there is a lot of enthusiasm for the Opposition.

"People are longing to see the change," said Meas Ny, an independent political commentator in Cambodia.

"Through Facebook people can get information instantly every day … so it builds the capacity of people to see that change is needed for their country and their society."

While the CPP hands out free T-shirts and cash gifts to lure voters, a feature of opposition rallies has been a tuktuk carrying a well-stuffed donation box, as working-class supporters help fund the party dollar-by-dollar.

Supporters of opposition party CNRP march during the last day of campaigning ahead of the elections. ( AP: Heng Smith )

Military supports ruling party 'forever'

One of the key features of this election campaign has been the role of the military, with any thin pretence of neutrality dropped altogether.

"[The military will support] Hun Sen, the President of the Cambodian People's Party, as Prime Minister of the Kingdom of Cambodia for the next electoral mandate — and forever," said Chea Dara, the deputy commander in chief of the armed forces.

In case anyone missed the message, Cambodia's Defence Minister Tea Banh threatened to "smash the teeth" of anyone protesting if the CPP wins on Sunday.

There are also allegations soldiers are being strategically deployed and illegally registered to vote in marginal seats.

The Phnom Penh Post newspaper reported that 300 soldiers were recalled from their border base in Preah Vihear province and registered in Srayang Commune, which the Opposition won by just 92 votes in 2012.

It's perhaps not surprising the army leans towards the ruling party — in 2015 the Government made almost every commander of a brigade, regiment or division a member of the CPP Central Committee.

"I think they genuinely believe that losing power would cause the country to spiral back into some form of conflict … and that by taking a firm stance to quash that threat, they're doing the right thing," said Mr Strangio.

On the positive side — ignoring dubious registration of soldiers — the voter lists that were so often the root of electoral irregularities in the past are reportedly much improved this year.

Facing up to Generation Facebook

It's perhaps telling that none of the young people the ABC spoke to actually went to Hun Sen's rally — but they all tuned in via a live Facebook feed.

Both sides have battled to capture the Facebook audience, the media wild card in a country where the CPP has traditionally owned most radio, TV and newspapers.

"A new young generation of voters has entered the arena … [and] these are young people that have no experience of civil war or upheaval," said Mr Strangio.

Cambodian Prime Minister Hun Sen is the world's sixth longest-serving premier. ( Supplied: The Phnom Penh Post )

One of those new generation voters is Phea, who makes a potent image of new-meets-old as he skateboards around the Socialist-style Vietnamese-Cambodian Friendship Monument in the capital.

He's 24 years old: the median age in this youthful nation.

"I already have [the candidate] in my mind because choosing the right leader will also affect your family," Phea told the ABC, being careful not to give away his preference.

He is on semester break from RMIT University in Vietnam and his wider experiences has shown him that Cambodia is still a country in transition.

"I really want to see Khmer kids developing, changing their behaviour — I think they still have low education," said Phea, who says he was one of Cambodia's first skateboarders.

Like the political leaders and the thousands who have rallied in recent days, the young man understands the importance of today's vote as a scene-setter for next year's national election.

"You have to really think carefully because different people might do different things for your country," he said.