The United Nations on Wednesday said Cambodia's general elections were held in a severely restricted environment surrounded by threats and intimidation that created a chilling effect.

UN human rights chief Volker Turk slammed the circumstances behind Sunday's vote as the victor, Prime Minister Hun Sen, announced he would resign and hand power to his eldest son after almost four decades of hardline rule.

Turk said the elections were conducted in a severely restricted space that negatively impacted on the rights of Cambodians to participate fully and equally.

Restrictive laws and policies hampered the registration and participation of opposition parties and candidates, while opposition parties, trade unions, NGOs and media were targeted through criminal and other legal processes, threats, intimidation, and on occasion physical attacks, he said.

"Cambodia has witnessed a constant shrinkage of democratic space in recent years, undermining fundamental freedoms and the right to participate in public affairs," Turk said in a statement.

Hun Sen, one of the world's longest-serving leaders, won 82 percent of the vote against no meaningful opposition.

The former Khmer Rouge cadre has run the kingdom since 1985, eliminating all opposition to his power, with opposition parties banned, challengers forced to flee and freedom of expression stifled.

The social media accounts of three media organisations deemed critical of the government were blocked before Sunday's election.

"I urge the government to address shortcomings, rectify deficiencies and engage in dialogue with all political parties and civil society actors to create an enabling and inclusive civic space for all Cambodians."

AFP