By Prak Chan Thul

TUOL TACHEN, Cambodia (Reuters) - After 28 years of rule by Hun Sen, Asia's longest-serving prime minister, the rallying cry for Cambodia's opposition ahead of next week's election is obvious, and it appears to be attracting an unprecedented number of voters.

"Change, change!" senior opposition politician Kem Sokha shouted through a microphone to a crowd of several hundred villagers in the southern province of Takeo. "July 28 is the day that we will end this dictatorship regime."

Kem Sokha's prediction is far-fetched, even though the crowd of flip-flop clad farmers roared their approval.

Hun Sen, a charismatic former commander in the genocidal Khmer Rouge, has built up a formidable electoral machinery through his ruling Cambodian People's Party (CPP) that easily outguns the opposition. Adding to his advantages are a compliant media, the CPP's deep pockets, and an election system that is prone to government meddling.

Hun Sen, 60, is a master of cultivating patronage networks and alliances within the CPP. The authoritarian leader has vowed to stay in power until his planned retirement in his mid-70s, and appears intent on building a political dynasty by promoting his three U.S. military-trained sons to top positions in the CPP and the army.

Still, the newly unified opposition hopes to mount its strongest challenge to the CPP since democracy was fully restored in 1998.

Under Hun Sen, Cambodia has transformed from a war-torn basket case into one of Southeast Asia's fastest growing economies, helped by a burgeoning garment export industry and growing political and investment ties with regional power China.

But the breakneck economic growth has been accompanied by a rise in social tensions over poor factory conditions and rural land rights in the country of 14 million, where a third of people live on less than 65 U.S. cents per day.

In rural areas, where more than 90 percent of Cambodians live and which are a backbone of CPP support, there is growing anger over huge land concessions awarded to foreign companies and which have benefited close political allies of Hun Sen.

Om Vanthoeun, a 61-year old farmer who attended Kem Sokha's rally in the village of Tuol Tachen, said he was most concerned about land grabs and what he said was an influx of immigrants from neighboring Vietnam.

"I just want change, even a little child wants change," said Om, who plans to vote for the opposition despite what he said was intimidation by his local CPP candidate.

HUN SEN'S SONS

While there are no reliable national opinion polls, most political analysts believe the CPP is on course to retain its majority but may lose ground from the 90 seats it currently holds in the 123-seat parliament. The two main opposition parties joined forces last year and are expected to improve on the combined 29 seats they won at the last elections in 2008.

The opposition received a morale boost on Friday when opposition leader Sam Rainsy returned after four years in exile in France. Sam Rainsy was given a royal pardon that removed the threat of a jail term, amid mounting pressure from the United States and other foreign donors to ensure the election is fair. Donors provide nearly half of Cambodia's budget.

On a visit to Phnom Penh last November, U.S. President Barack Obama told Hun Sen he should make sure the election was free and fair and pressed him to improve human rights, in what U.S. officials described as a tense meeting.

Tens of thousands of cheering supporters met Sam Rainsy, a former finance minister, who chose exile in 2009 rather than face trial for what rights groups said were politically motivated charges.

Some analysts believe the opposition could win enough seats to create political deadlock that would force Hun Sen into a coalition government with Sam Rainsy. Most, however, see the CPP keeping a strong enough majority to rule alone.

"Sam Rainsy's royal pardon was long expected and part of a well-rehearsed government strategy to clamp down on the opposition ahead of the polls, and then make last-minute targeted concessions to appease foreign donors," said Giulia Zino, Southeast Asia analyst at Control Risks in Singapore.

"The CPP is almost certain to secure a large parliamentary majority at the polls, ensuring broad policy continuity and basic political stability for foreign investors."

CPP lawmaker Sok Eysan denied that Hun Sen was favoring his sons by moving them into increasingly powerful positions.

"The party has a policy of promoting the role of youths, including some who were educated in the West and locally. We take care of everybody, youth is the next generation," he said.

Hun Sen's youngest son, 30-year-old Hun Many, is running for parliament for the first time in this election. He is already the deputy chief of cabinet and the head of a national student movement. Second son, General Hun Manith, is the deputy head of Cambodia's Intelligence Unit.

The eldest, General Hun Manet, is Hun Sen's favored son, who he has publicly suggested possesses divine powers. Hun Manet is a deputy commander of Hun Sen's personal bodyguard and head of the National Counterterrorism Taskforce.

"They are like the old guard except they have more education," said Henri Locard, a French historian who teaches at the Royal University of Phnom Penh. "If things continue as today, you will never see the end of Hun Sen."

(Writing by Stuart Grudgings; editing by Raju Gopalakrishnan)