The newly united opposition made significant gains in Sunday's polls, which independent poll monitors also criticised as flawed.
Hun Sen's Cambodian People's Party (CPP) announced yesterday it had taken an estimated 68 out of the 123 seats in the lower house, against an increased 55 for the Cambodia National Rescue Party (CNRP).
"We can say we've won this election," CPP spokesman Khieu Kanharith told AFP.
The CNRP however called for a committee representing the two main political parties, the National Election Committee, civil organisations and the United Nations to "be urgently established" to investigate complaints.
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The CPP had 90 seats in the previous parliament, so if confirmed the result would mark the loss of 22 seats, despite the exclusion of the opposition leader who was barred from running.
The CNRP has 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.
Rights groups also expressed concern that the ink used to mark voters' thumbs to ensure they did not vote more than once could be easily washed off.
"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.
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.