Names were missing from voter lists and some voters found others had used their ballot, reports said.

Prime Minister Hun Sen's ruling Cambodian People's Party says it has won by 68 seats to the opposition's 55.

However, Cambodia's opposition appears to have made a strong showing, reducing the ruling party's majority significantly.

"The Cambodia National Rescue Party (CNRP) cannot accept the results of the fifth parliamentary election... because the CNRP has found a lot of serious irregularities," the opposition party said in a statement on Monday.

It has called for a committee with members from both parties, the UN and the National Election Committee to "be urgently established" to investigate.

On Sunday, some voters told the BBC they could not find their names on voting lists, and said that indelible ink used to indicate that someone had voted was easily washed off.

"There are too many irregularities with far-reaching implications," opposition leader Sam Rainsy told a news conference.

"We're not seeking to bargain with the government. What we want is to render justice to the Cambodian people so their will is not distorted or reversed as before," he added.

Kol Preap, executive director of Transparency International Cambodia, told AFP news agency on Sunday that it was "very difficult to proclaim this a free and fair election".

"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," he said.

The Cambodian Human Rights and Development Association also said that there had been voting irregularities but said it was not clear whether they had affected the overall outcome of the polls.

"There were also a lot of people voting on a voter's behalf. But we do not know how much this affected the percentage when it came to the election results," Thun Saray, the association's president, said.

However, Cambodia's National Election Committee (NEC) said there had not been voting irregularities, AFP reported.

A spokesman for the ruling CPP, Khieu Kanharith, said the party would "follow the NEC's decision".

'Worst performance'

Hun Sen has been in power in Cambodia for nearly three decades, and the CPP had been widely expected to win the election.

However, if the provisional poll numbers are confirmed it would be the CPP's worst performance in 15 years.

The party enjoys considerable support in the countryside - in part due to the economic growth achieved there after the devastation caused by the Khmer Rouge regime in the 1970s, which was responsible for one of the worst mass killings of the 20th Century.

However, younger voters were thought more likely to look for a change and back the opposition and its leader Sam Rainsy, who recently returned to Cambodia from self-imposed exile.

In 2010 Mr Rainsy was sentenced in absentia to 11 years in prison, on a series of charges he says were politically motivated.

But analysts say his return to the country early in July - after a royal pardon was issued at Hun Sen's request - seems to have helped his party's cause.