Many Cambodians returned to their home villages to vote
The ruling Cambodian People's Party (CPP) is claiming victory in Sunday's parliamentary elections.
If confirmed, the result will extend the rule of Prime Minister Hun Sen - who has been in power since 1985 - for another five years.
A victory for his party had been widely anticipated in the 11-party election.
Cambodia has enjoyed strong economic growth and Hun Sen's tough stance over a border dispute with Thailand is said to have been popular with voters.
"We won the election. We are leading in most of the provinces," party spokesman Khieu Kanharith told AFP news agency.
The opposition Sam Rainsy Party (SRP) said preliminary results suggested it had taken 40 seats.
Final election results are expected to be announced in about 10 days.
Earlier Mr Kanharith had predicted the party would win 80 of the 123 seats in parliament, just shy of a two-thirds majority.
The CPP currently holds 73 seats and the SRP 24.
The BBC's Guy De Launey, in Phnom Penh, said the one major dispute of the election was when the SRP claimed thousands of its supporters had been unable to vote.
They believed they had registered but when they want to the polling stations they found their names were not on the electoral roll.
Despite this, monitoring organisations said that the vote had run far more smoothly than previous elections.
The prime minister has gained huge approval for taking a tough stance in an ongoing dispute with neighbouring Thailand.
The countries are arguing over land surrounding an ancient monument.
Both nations have massed troops near the 900-year-old Preah Vihear temple - which belongs to Cambodia.
Each claims the land surrounding the temple, and the dispute has raised nationalist fervour in Bangkok and Phnom Penh.
"People are more focused on the border issue at Preah Vihear temple than on the election," Hang Puthea, who heads an election-monitoring body, told AFP news agency.
Cambodia is due to hold talks with Thailand on the issue on Monday.
Hun Sen staged a coup in 1997, but has also won several elections
Under Hun Sen, Cambodia has achieved high economic growth - helped by revenue from the garment and tourist industries.
But the country is also experiencing soaring inflation and there is growing discontent over endemic corruption - both of which could have helped opposition leader Sam Rainsy, correspondents say.
The SRP has a strong power base in the capital but is weaker in rural areas.
The CPP has been ruling in a coalition with the royalist party, Funcinpec, which holds 26 seats.
A 2006 constitutional amendment means the CPP requires only a simple majority - rather than a two-thirds majority - to rule in its own right.
So all the indications are the CPP will be returned to office, with no need for a coalition partner.
Tens of thousands of Cambodia's eight million registered voters left the cities to vote in their home villages.
A ban on alcohol was enforced to ensure a peaceful poll - the fourth since decades of civil war ended.
Previous elections have been hit by violence. Scores of people - mainly opposition supporters and activists - were killed or beaten in the run-up to elections in 1998.
Campaigning for Sunday's vote passed off comparatively smoothly, but rights groups have flagged up ruling party control of the media and intimidation of opposition activists as continuing problems.
"Cambodian politicians and party activists know the CPP will use violence if necessary - which means the ruling party doesn't need to do so," said Brad Adams, of US-based Human Rights Watch.
International and domestic monitors scrutinised the voting, which began at 0700 (0000 GMT) and closed eight hours later.