Cambodia's opposition on Wednesday threatened a nationwide strike to protest at strongman premier Hun Sen's disputed return to power, as a UN envoy warned the nation was at a crossroads.

Sam Rainsy, whose opposition Cambodia National Rescue Party (CNRP) is boycotting parliament, said the country had returned to a one-party system through a "constitutional coup".

"So the whole country -- for one day we call for a strike at all factories. All civil servants, all shopkeepers will stop working on that day. This is one possible idea," Rainsy said at a news conference.

Cambodia's biggest labour organisation quickly threw its weight behind the proposed action.

"If the opposition holds a strike, the workers will join them," said Chea Mony, president of the Free Trade Union, which has 100,000 members.

"We want change because there are a lot of injustices and the government's promises to raise workers' salaries are empty," he told AFP.

Rainsy said further public demonstrations were "on the cards" unless there is an independent investigation into July elections that the CNRP alleges were marred by widespread vote-rigging.

"We do not recognise the government," he said. "It robbed power from the people."

His comments came a day after the legislature -- with only ruling party MPs in attendance -- reappointed Hun Sen for another five-year term as prime minister, extending his nearly three decades in power.

The United Nations special rapporteur on human rights in Cambodia, Surya Subedi, expressed deep "regret" at the decision to open parliament without the opposition, in comments in Geneva on Tuesday.

He slammed "indiscriminate and excessive" use of force during three days of mass protest in the capital earlier this month, which saw one protester shot dead and several wounded as security forces clashed with a stone-throwing crowd.

Noting further accusations of crackdowns on peaceful protests over the weekend, Subedi urged the authorities, "in the strongest possible terms, to refrain from further use of violence".

"Having seen the authorities, who exercised considerable restraint for weeks, again resorting to old tactics that violate basic civil rights in the past few days, I believe it is clear that Cambodia today stands at a crossroads," said Subedi.

He added that the opposition had "harnessed and focused the popular will for change in a way that has never before been seen in Cambodia's modern history", but warned that the party's success had yet to be measured.

In a televised cabinet meeting, Hun Sen threatened Wednesday to release a voice recording from a meeting with Rainsy earlier this month in which he said the opposition leader had dropped his demand for a probe and recognised the ruling Cambodian People's Party (CPP) won 68 seats ahead of the CNRP with 55.

"If he organises more demonstrations, (we will) play the recording or post it on Facebook ," Hun Sen said.

The 61-year-old former Khmer Rouge cadre -- who defected and oversaw Cambodia's rise from the ashes of war -- has ruled for 28 years and vowed to continue until he is 74.

Opposition leader Rainsy returned from self-imposed exile in July after a royal pardon for criminal convictions he contends were politically motivated, but he was barred from standing in the polls later that month.

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