Cambodia's opposition leader on Saturday denounced signs of voter fraud on the eve of elections and labelled Prime Minister Hun Sen a "coward" for not allowing him to participate.

Sam Rainsy said his Cambodian National Rescue Party (CNRP) had uncovered irregularities such as tens of thousands of duplicated voter names that would allow some people to cast ballots twice in Sunday's polls.

He also alleged that the ink used for voting could be washed off.

"We are going backward in terms of election fairness," Rainsy told reporters.

"More people will vote for us," he said. "But I suspect the ruling party, knowing this, will cheat more, will cheat like mad."

Local and international rights groups have also voiced concerns about reports of irregularities, while the UN special rapporteur on the situation of human rights in Cambodia, Surya Subedi, urged the kingdom to probe complaints.

"Every eligible voter must be given an equal opportunity to exercise his/her right to vote without intimidation, fear or pressure," he said in a statement. "Allegations of intimidation must be promptly investigated by the authorities."

But the ruling party Cambodian People's Party (CPP) dismissed the CNRP's allegations.

"They must point out those irregularities, not just talk. If they just talk, it is merely an excuse for the CNRP's coming election loss," CPP and government spokesman Khieu Kanharith told AFP.

Rainsy returned to Cambodia on July 19 from self imposed exile in France after receiving a surprise royal pardon for criminal convictions which he contends were politically motivated.

But he is barred from running as a candidate since the authorities said it was too late to add his name to the electoral register.

Several attempts to reverse the decision were rejected last week.

"If the prime minister wants to keep his position he must be brave enough to confront me," Rainsy said.

"It's very unfair and shows that the current prime minister is really a coward... The ruling party is nervous. That's why they block me by all means," Rainsy added.

"Any victory under such circumstances is worthless."

Rainsy, seen as the only major challenger to Hun Sen, had faced a total of 11 years in jail but was pardoned by King Sihamoni earlier this month at the prime minister's request.

Rainsy was stripped of his parliamentary seat in 2011 and removed from the electoral register late last year.

Last month all 28 opposition MPs were stripped of their status by a committee made up of ruling party members, which accused them of violating parliament's internal rules by joining forces to form a new party.

Unlike Rainsy, they are still allowed to take part in Sunday's election.

Hun Sen has been in power for 28 years. His CPP won the last two polls by a landslide amid allegations of fraud and election irregularities.

It is widely expected to secure another victory on Sunday, helped by its near-total control of the country's newspapers and television and a deep campaign war chest.

But experts say the opposition -- galvanised by the merger of two parties and Rainsy's return -- could reduce the CPP's parliamentary majority. The ruling party now has 90 seats in the 123 seat National Assembly.

Hun Sen's government is regularly accused of suppressing political freedoms and clamping down on dissent. In May Hun Sen said he would try to stay in power for another decade.

"Tomorrow, the election is not the end of our fight. It will be the beginning of the fight for real democracy," Rainsy said.