Cambodia's strongman Prime Minister Hun Sen on Friday vowed to establish a government under his leadership despite allegations by the opposition party of massive electoral fraud.

Hun Sen's ruling Cambodian People's Party (CPP) on Sunday claimed it had secured an estimated 68 of the 123 lower house seats, shading the Cambodia National Rescue Party (CNRP) which won 55.

The CNRP has rejected the results, citing widespread voting irregularities, and has vowed to press for nationwide protests unless an independent probe is launched.

According to the Cambodian constitution the kingdom's National Assembly must have "at least 120 members" and hold its first session within 60 days of the election, leaving the CPP well short by its own reckoning of the seats if the opposition chose to boycott parliament.

But in a defiant radio address on Friday the premier said his party has an "absolute majority" and will "proceed to set up the leadership of the parliament and government. They will not wait for you (the opposition)".

"There will be no stalemate... I am the Prime Minister. I will continue to be the Prime Minister," he added.

"The law does not specify how many lawmakers are needed for the first meeting."

He also urged the CNRP to join talks over participating in the next parliament.

The first parliament meeting will be convened by late September.

The CNRP has already called for a United Nations-backed investigation into allegations that 1.25 million legitimate voters were culled from the electoral roll, while more than one million "ghost names" and a further 200,000 duplicated names were added to the list.

Rainsy has said his party had in fact won a majority of 63 seats and repeated a vow to prevent the CPP "stealing victory".

In a statement, the opposition said its leader Sam Rainsy will announce the "CNRP's stand related to election results" on Tuesday.

Hun Sen, 60, a former Khmer Rouge cadre who defected from the murderous regime, has vowed to rule until he is 74.

He oversaw Cambodia's transformation from a nation devastated by the "Killing Fields" genocidal era in the late 1970s to become one of Southeast Asia's most vibrant economies.

The premier -- who has been in power for 28 years -- is regularly accused of ignoring human rights and muzzling political freedoms.