Cambodian Prime Minister Hun Sen warned Friday against any questioning of the legitimacy of his controversial reelection after a top UN official doubted whether the polls could be regarded as democratic.

Hun Sen, who has been in power for more than 33 years, saw his Cambodian People's Party win all 125 seats in July's parliamentary elections after a shuttering of media outlets and jailing of political opponents and journalists.

In a report to the UN Human Rights Council earlier this week, UN Special Rapporteur for Cambodia Rhona Smith said the elections had "consigned multiparty liberal democracy to history for the next five years."

But speaking from the floor of the United Nations General Assembly in New York, Hun Sen warned against any outside "interference" in Cambodian politics.

"The free choice of the Cambodian people and the legitimate result of this election is not a subject for question or debate," the veteran strongman said in his speech.

"Some external circles, however, who are on a mission to interfere in the domestic affairs of Cambodia still fail to see the quality and integrity of our election process by issuing statements against or attacking the election outcome.

"Such actions are a serious assault on the will of the Cambodian people."

A loud crowd of protesters gathered outside the UN building to denounce the Cambodian leader, some holding signs and chanting, "Hun Sun is a traitor!"

In her report, Smith said the dissolution of the main opposition Cambodia National Rescue Party and ban a large number of other senior opposition figures from all political activity "seriously calls into question the genuineness of these elections."

But Hun Sen said he had transformed a country "previously famous for its killing fields" when the dictator Pol Pot oversaw a reign of terror that left up to two million people dead from starvation, overwork or execution.

"The Cambodian nation is enjoying the dividends of peace, stability and rapid development which had never been seen before in its modern history," he said.

"Cambodia now enjoys full peace and has become a popular tourist destination in Southeast Asia, a food exporter, an outstanding performer in poverty reduction and has seen an improvement of all social indicators."

The United Nations has played a key role in Cambodia's elections since the end of the Pol Pot era and the country's subsequent occupation by Vietnam in the 1980s, sponsoring the first democratic polls in 1993.