A few hundred people, including monks and elderly regime survivors, gathered early Friday at Choeung Ek -- the most notorious of the regime's "Killing Fields" on the capital's outskirts -- burning incense and saying Buddhist prayers at a memorial stupa housing the skulls and bones of victims.
The event commemorates the April 17, 1975 triumph of the hardline communist Khmer Rouge over the US-backed republican army of Lon Nol.
That warmth swiftly turned to horror as the cadres evacuated the city of two million people at gunpoint, in one of the largest forced migrations in recent history.
The sick, elderly and very young perished, their bodies littering the roadsides, as the "bourgeois" city dwellers were marched into the countryside to scratch a living from the parched, over-used soil.
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By the time the tyrannical rule of Pol Pot -- or "Brother Number One" -- was ousted four years later, an estimated two million Cambodians had been killed by execution, starvation or overwork as the Khmer Rouge drove the country back to "Year Zero" in a madcap agrarian peasant revolution.
"Forty years ago Pol Pot turned Cambodia into a hell -- a ghost land," she told AFP with tears in her eyes after lighting incense for her loved ones.
"I still hate that regime... Their sins are vivid in my eyes now. They starved us, jailed people with no food and water until they died... I saw them smash childrens' heads against a tree trunk."
Only after the regime was forced out by Vietnamese soldiers in 1979 did the scale of the its atrocities emerge, with the bones of thousands of victims uncovered at several mass graves, including at Choeung Ek.