Official results were expected later in the week.
Thun Saray, head of the Cambodian election monitoring group Comfrel, said it was "early to declare victory, but the trend shows that CPP is winning".
However, Sam Rainsy, the main opposition leader, dismissed the claims and called for a re-run of the polls in districts around the capital Phnom Penh.
"Neither party won more than two-thirds of the seats," he told reporters, estimating that no party received more than 70 seats, according to a tally by his supporters.
He also said that 200,000 of Phnom Penh's 722,000 voters had not been able to cast ballots because their names were missing from the electoral lists.
"We don't accept the result in Phnom Penh," Sam Rainsy, whose party held 24 seats in the last parliament, said. "I demand a re-run of the election in Phnom Penh to bring justice to voters."
"I call for a demonstration in Phnom Penh. I appeal to all people whose names were unfairly deleted - please hold a huge protest in Phnom Penh."
Election observers reported a number of cases of voters' names being removed from the lists, but they said they doubted the problem was as widespread as Sam Rainsy claimed.
"The atmosphere for the election day is better than past elections. But the most prominent point is that the turnout was low and a lot of names disappeared," Hang Puthea, head of the Nicfec group of election monitors, said.
But he added: "I can't believe that as many as 200,000 names went missing. I could believe the number is maybe 20,000."
About 17,000 domestic and international observers monitored the voting at more than 15,000 polling stations.
Previous polls held in Cambodia were marred by violence. Scores of people - mainly opposition supporters and activists - were killed or beaten in the run-up to elections in 1998.
Election monitors said political violence had diminished greatly but unequal access to the media was still a problem.
More than eight million people were registered to vote and 11 parties competed in Sunday's poll, the fourth since the end of the civil war.
Analysts had expected the vote to usher in a new term for Hun Sen, who has held control of the country for 23 years.
|Monitors said there were missing names from voter lists [AFP]
The former Khmer Rouge soldier's popularity at the polls was buoyed by a surge of nationalism amid a tense border dispute with neighbouring Thailand.
Voters appeared to rally around the 57-year-old who has a reputation for being a strong leader.
He had been expected to win the vote even before the military standoff escalated earlier this month, but patriotic passions over the Preah Vihear temple and Hun Sen's firm stance against Thailand swayed many undecided voters in his favour, analysts say.
Many voters said their overriding concern was the territorial dispute with Thailand.
"I will vote for those who can solve the issue of Preah Vihear temple immediately after they take power," Lam Chanvanda, a 56-year-old businessman, said as he stood in a long queue of voters.
"Before I was never interested in the border, but now it is in my heart."
Thousands of soldiers from both sides are facing off near the 11th-century Khmer temple. Foreign ministers from the two nations are set to meet on Monday in hopes of resolving the conflict.
"Everybody now supports the government because this is a national issue," Kek Galabru, a prominent Cambodian human rights activist and election monitor, said.