Cambodia's banned opposition movement has mounted a show of defiance against the Government with a mass noodle-eating effort.

Key points: Human rights groups say former opposition activists are being harassed and intimidated

Human rights groups say former opposition activists are being harassed and intimidated Cambodia's opposition was forcibly dissolved in 2017 and senior leaders went into exile

Cambodia's opposition was forcibly dissolved in 2017 and senior leaders went into exile The country is now a one-party state ruled by Prime Minister Hun Sen for 34 years

At least 1,500 opposition supporters were on Sunday expected to gather in noodle shops across the South-East Asian country, where they will tuck into num banh chok — a popular noodle soup dish — in what the opposition is calling "passive resistance" to a government crackdown.

Nearly 150 grassroots former members of Cambodia's opposition party, which was forced to disband after it was outlawed in 2017, have been interrogated, charged or arrested in recent weeks.

Many of them were targeted after gathering in groups for meals at local noodle restaurants over the past two months, in what the Cambodian Government sees as a cover for political meetings.

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"When they appear in court or at the police station the first question is: Why are you eating num banh chok?" exiled deputy opposition leader Mu Sochua told the ABC.

"And the answer is: Is that a crime? What stops us from eating noodle soup?

"Now, it is not just [about] eating noodle soup. It is defending your right [to] the freedom of assembly … and eating noodle soup, to us, is a form of passive resistance to oppressive measures."

Rival noodle-eating plans 'scare restaurants'

The country's authoritarian Government is now planning to hold a rival noodle-eating campaign on Sunday.

In a speech on Monday, Prime Minister Hun Sen called on members of his ruling Cambodian People's Party (CPP) to consume Khmer noodles on Sunday, to counter what he said was an opposition attempt to politicise the national dish.

"They want to turn the issue of num banh chok into a political matter, that anyone who eats num banh chok are all their supporters," local news outlet Voice of Democracy (VOD) reported him as saying.

"Even some num banh chok sellers are scared."

Ms Sochua compared Hun Sen weighing in on the issue to Australians feeling like they needed government approval before they felt safe to have a BBQ or eat a meat pie.

"Can you imagine if an Australian citizen had to ask permission from your Prime Minister to buy and eat a meat pie?" she said.

In late 2017, the courts forcibly dissolved the opposition Cambodian National Rescue Party (CNRP) and banned 118 of its most senior members from political activity for five years.

Many of them, including Ms Sochua, fled overseas after party leader Kem Sokha was arrested and charged with treason in September 2017.

His case has not gone to trial and he remains under house arrest in Phnom Penh.

Cambodia's opposition movement was left in tatters and unable to contest last year's election, an event widely condemned as a farce, in which Mr Hun Sen extended his 34-year rule without any notable competition.

Grassroots former opposition members, who remain under surveillance, appear to be the Government's latest target after they began meeting in noodle restaurants in an attempt to keep opposition networks alive.

"Anyone who even dares to mention the name CNRP is accused of supporting what they call the 'rebels' and can be arrested," said Ms Sochua.

Mu Sochua says Cambodian opposition supporters in the US and France will also take part in the noodle-eating day. ( Facebook: Mu Sochua )

"However, the grassroots members are very determined — they have not defected to the ruling party although Hun Sen has done everything possible to get them to defect.

"We are very, very proud of grassroot supporters and activists who are telling us that they are still strong."

Questioning, charges, and a death in custody

In the past two months, 147 former local CNRP officials and members have been the subject of what human rights groups say is a campaign of politically motivated harassment and intimidation.

Many of those were summonsed to court or questioned by police after meeting in groups in noodle restaurants.

At least six were charged with incitement in March after publicly backing senior opposition figure Sam Rainsy's plans to return to the country, local media outlets reported.

And one man, Tith Rorn, the son of a former local-level official, died in custody within 72 hours of being arrested in April over a 2010 misdemeanour.

Cambodian Prime Minister Hun Sen declared before last year's election his intention to stay in office for another decade. ( AP: Heng Sinith )

Local rights groups claim bruising was visible on his body, and his family is calling for an investigation.

Cambodian Government spokesman Phay Siphan told the ABC that the decision to investigate Mr Rorn's death would be a matter for the courts.

He also denied that the questioning of opposition activists was ordered by the Government.

"The courts [had] suspicion on those people of gathering to have a plan to stand up against the Government as well [as] against the Prime Minister," Mr Siphan said.

Frivolous spat masks seriousness of issue

Human Rights Watch deputy Asia director Phil Robertson said that despite Hun Sen's absolute grip on power in Cambodia, the Prime Minister was clearly nervous about his authority being threatened — apparently even by something as trivial as noodle soup.

"PM Hun Sen and his authoritarian Government want the CNRP forgotten, so they are not at all happy that these local councillors are meeting to eat noodles together," he told the ABC.

"The Government is really blowing this all out of proportion, showing just how uncertain they are after the July 2018 sham election and the lack of international recognition it received."

While Australia's Government has given no indication that it plans to impose sanctions, the United States has imposed sanctions on some Cambodian officials, which it is considering expanding to more high-level Government figures as well as the possibility of wider trade sanctions.

The European Union has begun a process that could see the country's tariff-free trade access to the lucrative EU market suspended next year.

And while that could cripple Cambodia's garment industry — which accounts for an estimated 80 per cent of the country's exports — Hun Sen has China in his corner, and therefore likely feels he can afford to direct his attentions elsewhere.

In his speech reported by VOD, Mr Hun Sen said his party would match any food-based attempts to mobilise resistance to the Government by eating the same Cambodian specialties.

"Next time, if you make an announcement about eating num krok [rice cakes], I will also eat num krok. If you make an announcement to eat banh chhev [stuffed savoury pancakes], I will also eat banh chhev."

