JIM MIDDLETON, PRESENTER: As Cambodians go to the polls this weekend, the country's long-serving prime minister is facing one of his toughest challenges yet.
Hun Sen's Cambodian People's Party is up against a united opposition, led by Sam Rainsy, who recently returned from years of exile.
But local and international election monitors have raised concerns that the election is neither free nor fair.
Auskar Surbakti reports from Phnom Penh.
(Footage of political rally plays)
AUSKAR SURBAKTI, REPORTER: After almost 30 years of the one prime minister, these Cambodians are campaigning for change.
They're supporters of the Cambodian National Rescue Party, known as the CNRP, which is the united opposition force that's reinvigorating hopes of a new government.
DUONG SOWAN, CNRP SUPPORTER (translation): I support the CNRP because it has good policy: fighting corruption, providing good education to students, and giving freedom to the youth to speak freely anywhere.
AUSKAR SURBAKTI: The CNRP is being led by Sam Rainsy who returned from France just last week after four years of self imposed exile.
In 2010 he was sentenced to 10 years jail in absentia after being found guilty of manipulating a map to suggest that Cambodia was losing land to Vietnam. Charges he says were politically motivated.
(Footage of political rally with Sam Rainsy plays)
The Cambodian government had come under pressure from the United States to allow Mr Rainsy to return, culminating in a royal pardon that paved the way for his homecoming.
SAM RAINSY, CAMBODIAN OPPOSITION LEADER (translation): I'm excited, I am very happy to be back to meet all of you today and I thank you all for coming here.
Now we will travel to the city together and I am here to rescue our nation together.
AUSKAR SURBAKTI: While Mr Rainsy has been barred from running in this election, because his name was removed from the electoral register, his presence has galvanised the opposition calls. It's also prompted government supporters to act.
(Footage of CPP rally plays)
His return has created a sense of real competition for the ruling Cambodian People's Party, or the CPP, led by the prime minister, Hun Sen, who has been in power since 1985.
TAM TIVITA, CPP YOUTH (translation): I support this party because prime minister Hun Sen has produced so many good things for the country, especially the development in Phnom Penh.
AUSKAR SURBAKTI: The CPP has increased its majority in parliament at every election since democracy was fully restored in 1998, winning 90 out of 123 seats at the most recent election in 2008.
Its decades in power have resulted in a captive government bureaucracy, and most of the media is state controlled, giving the party a significant edge in this year's vote.
Even with a reignited opposition party, the CPP is confident it will win the election.
PHAY SIPHAN, CAMBODIAN GOVERNMENT SPOKESMAN: The independence organisation, it showed that the 80 per cent of the people support prime minister Hun Sen as well as the CPP. So from this result we understand that we can do, I mean win landslide in this voting.
AUSKAR SURBAKTI: Cambodia has enjoyed relative peace and state under Hun Sen, as well as enviable economic growth. Last year, GDP (Gross Domestic Product) grew by more than 7 per cent.
It's something the opposition finds hard to dispute. But it does argue that most people are missing out on the benefits of the boom.
SON CHHAY, OPPOSITION MP: The current government is so corrupt. They've been doing everything to benefit themselves. So after serious study we found that at least $1,300 million to $1,500 million a year has been embezzled by this corrupt government.
AUSKAR SURBAKTI: It's an accusation the government strongly denies, but with around 30 per cent of Cambodians living on less than $1 a day it's a widely held belief.
This election has managed to mobilise hundreds and thousands of supporters of both the government and the opposition like no other campaign in the past, with many of those turning out young people. With around a third of all registered voters aged between 18 and 30, both parties are keen to capture the youth vote.
But there are concerns that this year's election won't be entirely free and fair. Cambodia's Independent Committee for Free and Fair Elections, or COMFREL, says the vote will be the country's worst.
KOUL PANHA, COMFREL: Around 13.5 per cent of the registers voter miss their name on the vote list because they have some error name in the voter list, or they lift their name from the voter list.
AUSKAR SURBAKTI: There are also reports of hundreds of thousands of phantom voters on the electoral role, with nearly all of Phnom Penh's communes registering more people than there are actual voters.
Election observers are allowed to monitor polling stations but COMFREL says it simply does haven't the capacity to monitor all 19,000 locations.
The Cambodian Government is under international pressure to hold free and fair elections, with some US politicians threatening to cut aid if the polls are not transparent. And there are fears of a backlash from local government and opposition supporters if this election is seen to be unfair.
KOUL PANHA: I appeal to all people, Cambodian people, should restrain the action of violence, they should not engage with violence, they should be settle conflict, the problem through the peaceful means.