Suy SE - When black-clad Khmer Rouge soldiers first charged into Phnom Penh they were welcomed with cheers, remembers Chhung Kong, a teacher in the Cambodian capital during the seventies.

Few foresaw the horrors that lay ahead as Pol Pot’s victorious communist army seized control early on April 17, 1975, signalling the end of a bloody civil war, and ordered the city’s two million people to evacuate.

Four years later Chhung had lost 16 of his relatives to the regime, Phnom Penh was deserted and his school transformed into the Tuol Sleng, or S21, torture chamber - one of the most grotesque emblems of a paranoid rule that wiped out a quarter of Cambodia’s population.

On Friday the capital will mark the 40th anniversary of the day Pol Pot defeated US-backed republican Lon Nol and began a tyrannical rule that saw the Khmer Rouge reset the kingdom to “Year Zero”.

Memories of the bloody agrarian revolution, which left up to two million dead from starvation, overwork or execution, remain seared in the mind of Chhung, now 71. “At first people were cheering, waving flags... we didn’t think about the death and danger that was coming,” he says sitting inside his office in Phnom Penh, where he now practises as a lawyer. “About 10 or 11am the same day, the Khmer Rouge soldiers ordered us off the streets and to start leaving the city. They said it was for only a few days.”

The summer sun was strong, Chhung recalls, and many of the soldiers had one trouser leg rolled up to the knee. Others coralled the evacuees by firing AK-47s into the air, strings of extra bullets across their chests. Tens of thousands of people were marched for days out of the city, some carrying the very young while others pushed the sick along on hospital beds, and soon Chhung started to see bodies littering the roadsides.

A few days later Phnom Penh was empty, its entire population relocated to work in the countryside in one of the largest forced migrations in modern history.

World turned upside down

The next four years of Khmer Rouge rule saw Cambodians turn on each other in a desperate bid to survive as Pol Pot’s socialist agrarian ideal swiftly unravelled and the nation was reduced to a workhouse.

Those outside the regime’s cadres were left scrambling for survival, and near the capital the “Killing Fields” multiplied as the regime systematically destroyed anyone they feared would oppose them.

As a French teacher at a high school in downtown Phnom Penh, Chhung was deemed a class enemy of the peasant revolution. He was sent from the “bourgeois” city to work in a rural co-operative 30 kilometres (20 miles) away, digging water canals to irrigate the parched land.

Chhung says he survived by “luck”, unlike family members including his father, three brothers, sister, nephews and nieces who died from starvation.

Even today, he still refuses to enter his old school, its blood-stained floors preserved as a chilling testament to the thousands who died there after its conversion to notorious death camp S21.

“Now, I just drive by it... I never enter the place... Why? Because it is the place where I taught students... I still (want to keep) the feeling that it is a school, not a prison,” says Chhung, who never returned to teaching.

In 2010, a UN-backed war crimes court sentenced former Tuol Sleng prison chief Kaing Guek Eav, alias Duch, to 30 years in prison - later increased on appeal to life - for overseeing the deaths of 15,000 people. He was the first person to be held accountable for the regime’s crimes.

Last August the two most senior surviving Khmer Rouge leaders - Nuon Chea, 88, known as “Brother Number Two”, and former head of state Khieu Samphan, 83 - were given life sentences for crimes against humanity. Both have appealed.

Their two-year trial focused on the forced evacuation of Cambodians from Phnom Penh into rural labour camps as well as murders at one execution site.

No escape from history

In March, the court charged three more former Khmer Rouge members with crimes against humanity, ignoring warnings by strongman Cambodian premier Hun Sen - a mid-ranking regime cadre before he defected - that further prosecutions risk reigniting conflict.

Before civil war engulfed Cambodia in 1970, Phnom Penh was a gentle-paced capital of majestic French colonial-era buildings and soaring Buddhist stupas. Older residents remember a city where motorbikes and bicycle-rickshaw outnumbered cars.

Those who returned once the Khmer Rouge were defeated by Vietnamese forces in 1979 found a ghost town, a shell of the once bohemian Southeast Asian capital.

In recent years the city has rebounded as the driving force of a small but growing economy that many hope will lift Cambodians out of poverty.

Tourists throng to its old world charm, which is now complimented by coffee shops and trendy bars.

Reconciliation is central to that gradual recovery, says Youk Chhang, director of the Documentation Center of Cambodia, which researches the Khmer Rouge’s crimes. “We cannot escape from our history; however, we do not have to be enslaved by it,” he says.