Cambodians voted on Sunday in an election likely to hand another five years in power to Asia's longest-serving prime minister, Hun Sen, but an energised opposition says there have been irregularities and it will continue to fight for true democracy.

The opposition believes nearly a million names are missing from voter lists and has complained about the disruption of meetings, alleged vote-buying and campaigning by the security forces for Hun Sen, who defected from the genocidal Khmer Rouge and was first appointed premier in 1985.

"The partisanship of the military and police has created an intimidating atmosphere for voters in many parts of the country," U.S.-based Human Rights Watch said in a statement.

The ruling Cambodian People's Party (CPP) is confident of victory but political analysts believe a stronger opposition may dent its majority.

More than 9.6 million people are eligible to vote, with polls opening at 7 a.m. (0000 GMT) and closing at 3 p.m. (0800 GMT). Early results may come on Sunday evening.

The opposition Cambodia National Rescue Party (CNRP) has been buoyed by the return home of a popular party leader, former finance minister Sam Rainsy, after a royal pardon.

Sam Rainsy had faced the threat of a jail sentence handed down in 2010 for spreading disinformation and falsifying maps to contest a new border agreed with Vietnam, charges he called politically motivated.

He returned to Cambodia too late to register to run in the election, or even to vote, and the electoral authorities rejected his late request to do so.

But Sam Rainsy has campaigned strongly for the opposition, attracting large crowds, and has complained to international observers of irregularities in the running of the election.

"In spite of this uphill battle, any gain we make will be very significant. The election is not the end of our fight, it will be the beginning of the fight for real democracy," he said on Saturday.

Under Hun Sen, Cambodia has been 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 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.

Hun Sen's CPP is a formidable electoral machine, with millions more party members, that easily outguns the opposition. Adding to its advantages is a compliant media, its deep pockets, and an election system that is prone to government meddling.

"The CPP has 5.7 million members; the other party has only just over a million," said CPP lawmaker Cheam Teap, confident of victory in Sunday's ballot.

Hun Sen's CPP had 90 of the 123 seats in the outgoing parliament while the CNRP had 29.

CPP's Cheam Yeap said the ruling party would win between 88 and 92 seats, but some analysts believe the opposition has its best chance in years of denting the CPP majority.

"The opposition will gain more seats, judging by all the support we've seen," said independent analyst Chea Vannath.

"But regardless of how many more seats the parties get, they should be united for the country rather than confronting each other," she added, advocating a government of national unity.

Alcohol sales and political campaigning were banned for 48 hours from midnight on Friday to ensure calm.