PHNOM PENH,  (AFP) - Cambodia's strongman premier Hun Sen looked poised Sunday to extend his 28-year rule after elections marred by allegations of widespread irregularities and the exclusion of the opposition chief.

The former Khmer Rouge fighter turned prime minister appeared so confident of victory that he did not even bother personally to campaign.

The opposition decried what it described as the kingdom's worst ever poll irregularities, including missing voter names and thousands of people who turned up to find someone else had used their ballot.

"The situation is more serious than at any previous election," Cambodian National Rescue Party (CNRP) spokesman Yim Sovann told AFP.

Rights groups also expressed concern that the ink used to mark voters could be easily washed off.

Protests broke out at one polling station in the capital Phnom Penh where a crowd destroyed two police cars, military police spokesman Kheng Tito said, as anger erupted over names missing from the voter list.

"It is very difficult to proclaim this a free and fair election," said Kol Preap, executive director of Transparency International Cambodia.

"I think the level playing field in the process didn't really exist. There has not been equal access to the media and the opposition leader was not allowed to run as a candidate."

The National Election Committee denied  irregularities.

Even before polls opened, the opposition had said a Hun Sen win would be "worthless" without the participation of its leader Sam Rainsy.

The French-educated former banker returned to Cambodia on July 19 from self-imposed exile after receiving a surprise royal pardon for criminal convictions which he contends were politically motivated.

But he was barred from running as a candidate since the authorities said it was too late to add his name to the electoral register.

Rainsy toured polling stations in Phnom Penh on Sunday to "collect more evidence" of vote irregularities.

He said that if indications pointed to a "plot to rig the election" then "definitely we will protest".

Local poll monitor the Committee for Free and Fair Elections in Cambodia alleged that up to 1.25 million people who were eligible to cast ballots were not on voter lists.

About 9.6 million people registered to vote -- more than one third of whom are aged under 30. Early results were expected late Sunday.

A spokesman for Hun Sen's Cambodian People's Party (CPP) said the party was confident of another landslide. "We expect to keep an absolute majority," Khieu Kanharith said.

A smiling Hun Sen kissed his ballot and dropped it into the box at a polling station at a teacher training school in the town of Ta Khmao near the capital.

First in line to vote there was 87-year-old Yim Hor Eam who described it as a "historic day".

"I voted for peace for the country. If we make a wrong decision, the country will be in danger, will be destroyed," he told AFP, without revealing who he supported.

Hun Sen defected from the Khmer Rouge, and oversaw Cambodia's transformation from a nation devastated by their "Killing Fields" genocide era in the late 1970s to become one of Southeast Asia's most vibrant economies.

The 60-year-old premier -- who has vowed to rule until he is 74 -- is regularly accused of trampling on human rights and quashing political dissent.

For decades, Hun Sen's simple message -- that he and his party liberated Cambodia from the Khmer Rouge and ushered in decades of peaceful development -- has been enough to guarantee support.

"Hun Sen has been adept at humiliating his political opponents by stripping opposition deputies of their immunity and orchestrating the exile of Sam Rainsy," said Cambodia expert Carl Thayer, a professor at the University of New South Wales in Australia.

But he is "also a genuinely popular figure", he added.

"Because the opposition lacks deep and widespread support in the provinces their best prospect is to reduce the number of seats held by the CPP."

The ruling party now has 90 seats in the 123-seat National Assembly.

There are tentative signs of change as social media allow the opposition to reach out to young, urban voters.

Thousands of opposition supporters have turned out in recent weeks for campaign rallies, chanting "Change!"

While the opposition hopes  Sunday's election will be a step at least towards a shift in power, the premier appears to have other plans.

His three US-educated sons have been handed top party or army positions and the youngest is running in Sunday's election, fuelling speculation they are being groomed to replace him.