Cambodia’s main opposition has rejected the result of Sunday’s elections, which handed another win to longtime Prime Minister Hun Sen.

The ruling Cambodian People’s Party of Hun Sen claimed victory after winning by 68 seats to the opposition’s 55 in the lower house. Hun Sen, a former Khmer Rouge fighter, has now been in office for 28 years.

At a Monday news briefing, the Cambodia National Rescue Party (CNRP) said the weekend polls were marred by “serious irregularities.”

“The 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.

The BBC reports names were missing from voter lists and some voters found others had used their ballot while indelible ink used to indicate someone had voted was easily washed off.

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

As many as a million people may have been deprived of their right to vote, Rainsy said.

Rainsy called on the UN and the National Election Committee (NEC) to launch a joint investigation into “all serious election irregularities.”

The NEC said it worked hard to ensure the elections was fair. A spokesman for the ruling CPP, Khieu Kanharith, said the party would “follow the NEC’s decision.”