Prime Minister Hun Sen has warned opposition leaders they will face ruling party rallies at home if he is met with anti-government demonstrations during his high-profile trip to the US next month, a threat that raised the spectre of pro-government protests in October that ended with two opposition lawmakers in hospital.

The remarks came barely 24 hours before the expected arrival in Phnom Penh of US Secretary of State John Kerry, who flew in last night after visiting Laos and will today meet the premier, Foreign Minister Hor Namhong, Cambodia National Rescue Party acting president Kem Sokha and civil society representatives.

Hun Sen flagged the retaliatory rallies in a Facebook post responding to a message to his page by young Khmer-American opposition supporter “Brady Young” on Sunday.

Brady told Hun Sen that peaceful demonstrations were being prepared to coincide with his attendance at a US-ASEAN summit in California next month, to which US President Barack Obama invited ASEAN leaders in November.

Referring to himself as “uncle”, the prime minister told Young that his group could stage protests if they believed it useful but warned of a backlash, writing that Young’s “bosses inside and outside the country” must “recognise the rights that uncle’s supporters have to demonstrate against nephew’s leaders” in Cambodia.

“A possible demonstration in the country could occur to oppose the opposition leaders,” he wrote.

The comments echo remarks the premier made after being confronted by anti-government rallies in France and New York last year, when he alluded to a mass demonstration against CNRP deputy Kem Sokha.

The next day, October 26, a 2,000-strong pro-government rally materialised outside the National Assembly calling for Sokha to be stripped of his position as assembly vice president, an ousting which occurred soon after.

Some protesters from the rally then viciously attacked CNRP lawmakers Nhay Chamroeun and Kong Saphea as they tried to leave the building, inflicting serious injuries. Three soldiers, linked to the prime minister’s bodyguard unit, have since been charged over the attack.

The premier distanced himself from the violence, but acknowledged the rally was in response to the protests in France and New York, the latter of which he yesterday accused the CNRP of directly funding.

People protest against Kem Sokha in October at the National Assembly in Phnom Penh where two opposition lawmakers were attacked and hospitalised.Heng Chivoan

According to a State Department briefing on Kerry’s current Asia tour, which also includes high-level talks in China, the US is concerned about “recent political developments” in Cambodia.

“The relationship between the ruling party and the opposition party is fraught right now. The leader of the opposition party is out of the country, so Secretary Kerry will meet with not only the government but also the opposition,” said the briefing note.

“He’ll also meet with members of civil society to underscore both US support for democracy in Cambodia but also, importantly, US support for human rights, for civil rights, and for political space.”

The briefing also noted Cambodia’s chairmanship of ASEAN in 2012 left a “black mark” on the group, a pointed reference to the Kingdom’s unwillingness to risk the anger of major investor China by signing a joint statement on the contentious South China Sea issue.

Weeks after the October attack on the CNRP lawmakers, opposition president Sam Rainsy fled into self-imposed exile to avoid prison on charges widely considered politically motivated.

Responding yesterday, Rainsy said the premier’s “childish” remarks amounted to a confession that he initiated “all the recent political violence in Cambodia”.

He said the party had not organised protests against Hun Sen abroad, but respected the right of Cambodians in America to exercise their freedom of expression.

“The [protests abroad] are always peaceful and always abide by the laws, rules and regulations of the democratic countries where they take place and where there is real freedom of expression,” Rainsy said, via email. “The [CPP-organised protests] are just violent acts of state-terrorism whereby the government conducts its threats or reprisals in its policy of fear and intimidation.”

Emails seen by the Post show that some Khmer-American opposition supporters are planning “mass protests” to meet the premier in California. Yesterday, a CNRP fund-raising network in the US – CNRP USA – distanced itself from any protests being planned for Hun Sen’s visit, calling such demonstrations “unfortunate”.

Political analyst Ou Virak, who will meet Kerry today with other civil society representatives, said Hun Sen was trying to avoid being “embarrassed” on the world stage by pre-empting protests in the US. However, he added, the “serious threat” revealed his “true colours” and should be confronted head on by Kerry, whose visit will largely focus on trade and economic relations.

“[An invited visit to America] is something that [Hun Sen] has basically been desperate for . . . he doesn’t want it to be spoiled by some protests . . . In his view, any protests would be an embarrassment to him and to Cambodia and that’s why he views it with such animosity and such anger,” Virak said.

“John Kerry needs to stand up . . . The US should come out swinging strong, and defend that fundamental right to protest.”