The ruling party of strongman Cambodian premier Hun Sen claimed victory in Sunday's elections which were marred by allegations of widespread irregularities, but it faced rare competition from a resurgent opposition.

Although official results had yet to be announced, the prime minister's Cambodian People's Party (CPP) said it expected to take 68 out of the 123 seats in the lower house.

"We can say we've won this election," CPP spokesman Khieu Kanharith told AFP.

The CPP had 90 seats in the previous parliament, so if confirmed the result would mark the loss of more than 20 seats, despite the exclusion of the opposition leader who was barred from running.

The electoral authorities said final results could take weeks to compile.

Hun Sen, a former Khmer Rouge fighter, has been in power for 28 years. 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.

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.

The opposition caused brief confusion after claiming it had won the polls but it quickly retracted the statement.

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.

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

"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 said his party was still considering whether to accept the ruling party's claim of victory.

"We applaud the victory of the Cambodian people and the victory of democracy," he added.

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 were registered to vote -- more than one third of whom were aged under 30.

Soon after polls opened 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 whom he supported.

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

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.

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

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

While the opposition hoped the election would 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 ran in Sunday's election, fuelling speculation they are being groomed to replace him.