By Agence France-Presse
The 66-year-old prime minister has ruled Cambodia for over three decades, with the Southeast Asian country tilting towards China in recent years thanks to loans for infrastructure and few complaints on human rights issues.
His administration was renewed for another five years in July following elections critics condemned as a sham, inciting threats from the European Union to revoke duty-free access to EU countries.
But Hun Sen remained defiant Saturday as he inaugurated a monument with carvings showcasing his government's achievements.
"Don't make war by using what is called democracy and human rights, in which democratic countries used to make the mistake of supporting Lon Nol's coup," he said in a speech to thousands of officials.
Lon Nol's US-backed regime was ousted by the ultra-Maoist Khmer Rouge in 1975, jumpstarting four years of horror as Cambodians were forced to live on farming communes, endure hard labour, and were tortured and murdered if they were suspected of plotting against leader Pol Pot.
Hun Sen was a former Khmer Rouge cadre who defected and had a role in toppling the genocidal regime in 1979, although the US later sought to keep the ousted Khmer Rouge at the table in the United Nations.
"You as a democratic country ... supported Pol Pot, who used to kill people with no regards for respecting human rights," Hun Sen said, without naming the US. "You supported them to keep a seat at the UN."
Cambodia's government dissolved the main opposition party and jailed its leader ahead of July elections in which it won every seat in parliament, transforming the nation into a one-party state.