PHNOM PENH – At the Bonna Business Center, a tiny Internet cafe near the opulent mansion of Cambodia's long-ruling Prime Minister Hun Sen, coffee is served with a big lump of dissatisfaction.

"They talk about seven percent economic growth," says Ou Rithy, 27, who hosts weekly political discussions at the cafe with other young Cambodians. "But I'm still a poor man."

He blames Hun Sen's ruling Cambodian People's Party (CPP), which won a recent general election widely criticised as rigged but lost the nation's heart and soul - its restive, tech-savvy and increasingly outspoken youth.

About 70 percent of Cambodia's 14 million people are under 30, a demographic whose growing political clout is challenging the country's aging and corrupt leadership, while breathing life into a once-moribund opposition who have called for mass protests on Saturday.

The Cambodia National Rescue Party (CNRP) said Hun Sen cheated his way to victory in the July 28 election and has vowed to protest until an independent committee is formed to investigate alleged voting irregularities.

Hun Sen has denied the allegations.

According to initial results, the CPP won the election with a greatly reduced majority, revealing widespread unhappiness with Hun Sen's iron-fisted rule despite rapid economic growth.

CNRP leader Sam Rainsy has urged supporters to "pray for peace" at Saturday's protest, but many fear it could lead to months of political deadlock or even violence. In recent days, thousands of riot police armed with batons and shields have rehearsed crowd control methods in Phnom Penh's parks.

It is Hun Sen's biggest crisis in two decades, threatening to destabilise a tiny Southeast Asian nation with strong economic and political ties to China.

Cambodia owes its youthful demographic to its tragic past. Whole generations were wiped out during the 1975-79 "Killing Fields" regime of the Khmer Rouge, when more than a million people were killed or died of disease.

Hun Sen has long hailed himself and the CPP for rescuing the country from the ensuing years of chaos and poverty. But such appeals increasingly fall flat with young people born long after the Khmer Rouge's terror ended.

"Social justice"

"Young people want social justice, they want jobs, and they want a good education system," says Ou Rithy, a political science graduate who has watched many peers desert his home province of Pursat to seek work in neighbouring Thailand.

Soaring use of smart phones and the Internet have allowed young Cambodians to sidestep the government's strict control of television, radio and newspapers.

In 2008, when Hun Sen easily won the last election, only about 70,000 people had access to the Internet, according to government statistics. By last year, that number had soared to 2.7 million, helped by a similarly exponential rise in mobile phones. There are now more cellphones used in Cambodia - 19 million - than there are Cambodians.

Also accelerating communication since the last election is the Khmer-language version of unicode, a computing encoding standard used for different languages and scripts. This allowed Cambodian Internet users to easily write and share information in their own language.

"Even those who don't speak English can still create Facebook accounts in Khmer," said But Buntenh, 34, a Buddhist monk and blogger at the Bonna Business Center.

State media routinely ignore opposition rallies. But this news blackout encouraged many Cambodians to seek information from social media, usually with just a click on their phones.

Despite its large campaign budget, the CPP underestimated the frustrations of ordinary Cambodians and the opposition's growing popularity, says Ou Virak, president of the Cambodian Center for Human Rights (CCHR).

"It wasn't until shortly before election day that we started to see the impressive level of youth involvement," he said. "Even then people weren't sure to what extent this would impact the actual results."

Young cambodians don't want revolution

More than a third of the country's 9.6 million eligible voters are under 30, although many work abroad and don't cast ballots, said the Committee for Free and Fair Elections in Cambodia.

The CPP has limited youth appeal. Hun Sen is 61 and has vowed to rule until he is 74. Its youth wing is widely regarded as a political vehicle for Hun Sen's youngest son,Hun Many, 30.

The opposition CNRP has a more youthful image, with Koul Kanha, director of the Committee for Free and Fair Elections in Cambodia (Comfrel), calling it the "Facebook party".

Formed last year after the merger of two parties, the CNRP owed part of its resurgence to social media. Photos and video of alleged election fraud also went viral thanks to Facebook, YouTube and Twitter.

But the government didn't attempt to shut down the social network, despite many users posting critical - and sometimes racist and vulgar - remarks about CPP figures.

Hun Sen disavowed a Facebook page bearing his name after Sam Rainsy repeatedly taunted him for having fewer "likes".

Some analysts say the rise of a youthful opposition to Hun Sen could signal a "Cambodian Spring" similar to the popular but often ill-fated movements against authoritarian rulers in the Arab world.

Ou Rithy disagrees. "Young people don't want a revolution, they want evolution - a gradual change based on non-violence," he says.

Sam Rainsy has accused the CPP of colluding with the National Election Committee to steal 2.3 million votes from his party. He disputes results showing the CPP won 68 seats in parliament to the CNRP's 55.

Hun Sen has vowed to form a government despite the opposition's campaign.