His return comes ahead of the country's July 28 general election, as Robert Carmichael reports from Phnom Penh.
Presenter: Robert Carmichael
Speaker: Mu Sochua, Cambodian opposition MP
(Sound of flags being put together)
ROBERT CARMICHAEL: Sam Rainsy's return has provided a boost to the opposition Cambodia National Rescue Party, or CNRP, ahead of the July 28 poll. Here at one of the party's buildings in Phnom Penh, CNRP supporters are preparing hundreds of handheld flags with which to greet him.
The CNRP reckons 20,000 people will line the roads today to welcome Sam Rainsy, and predicts as many again will attend a rally he is expected to address later. The CNRP is seen as the only serious challenger among the seven parties competing against Prime Minister Hun Sen's Cambodian People's Party, or CPP, which is widely expected to win the election.
Sam Rainsy left Cambodia in 2009 ahead of two court cases that saw him handed an 11-year jail term for actions and comments he made over the contentious border with Vietnam. Last week's royal pardon, which some donors had sought, annulled those convictions. Despite that, he is still barred from running for a seat and is not allowed to vote.
The Cambodia National Rescue Party is an amalgamation of two opposition groupings: The Sam Rainsy Party and the Human Rights Party of Kem Sokha, who used to run a human rights organisation. Between them they hold 29 of parliament's 123 seats.
On Saturday the two men - who have had plenty of bust-ups in the past - will embark on a swing around Cambodia to drum up support. Here's opposition MP Mu Sochua:
MU SOCHUA: In the countryside, both of them will be everywhere in the next eight days. [They] go together to show unity - of the united opposition - to show that those two men [are] the leaders of the people who have put on the agenda the interest of the nation.
ROBERT CARMICHAEL: One concern for the opposition during campaigning will be Sam Rainsy's safety. In 1997 there was an infamous attempt on his life - a grenade attack in broad daylight in Phnom Penh that killed 16 people including one of his bodyguards.
Mu Sochua says the opposition has little choice but to be optimistic that Sam Rainsy will not be harmed.
MU SOCHUA: If we continue to live in fear and be surrounded by bodyguards, it's no longer the image of who Mr Sam Rainsy is or Mr Kem Sokha is. There are some essential steps that we take - we have a small, private bodyguards around them but we will not ask for bodyguards and police around them from the state because it will totally destroy the image of we, the fighter for justice.
ROBERT CARMICHAEL: With little over a week until voting day, all eight political parties are campaigning hard to convince people to back them.
The electoral playing field, though, is heavily tilted towards the ruling CPP, which has 90 seats in parliament.
There are worries too about dirty tricks, most notably over the voter registration list, which is thought to have one million ghost voters. And it seems a further one million people who ought to be on that list have had their names deleted. Combined, that amounts to 20 percent of the electorate.
However it is also the case that the CPP and Hun Sen are popular, particularly in rural areas. The ruling party's campaign is predicated on the twin issues of peace and development, both of which resonate strongly with voters.
Mu Sochua says the opposition's focus is on the need for change. She has noticed far less fear among people this time than in previous elections, and says that all around Cambodia CNRP supporters are chanting the opposition's mantra:
MU SOCHUA: Yu trao ta daw, daw, daw. Yu trao ta change, change, change. This is the song on the street of Cambodia right now, every day, starting at 4 o'clock. Yu trao ta daw, daw, daw. Yu trao ta change, change, change.
ROBERT CARMICHAEL: Observers say Sam Rainsy's return should help an increasingly confident opposition win more votes. Whether it will fulfil their goal of change - democratically upending a ruling party that has enjoyed more than three decades of uninterrupted power - remains to be seen.