When the Khmer Rouge emptied Cambodia's capital, hundreds of thousands of people, including children, pregnant women and hospital patients, were sent on a journey that many would not survive.
Cambodian woman Lay Bony, who lost 11 relatives including her husband and children under the hardline Khmer Rouge communist regime, pictured during an interview at her house in Phnom Penh, on March 7, 2012.
Nearly four decades later, the episode is the focus of legal proceedings against former regime leaders that could be the last hope for justice over the deaths of up to two million people -- a quarter of the population.
The evacuation of once-bustling Phnom Penh in April 1975 reduced the city to a ghost town in a matter of days. It was one of the largest forced migrations in modern history.
More than two million people were expelled from the capital at gunpoint and made to march to labour camps in the countryside as part of the Khmer Rouge plan to forge an agrarian utopia.
No mercy was shown to the those too weak to make the journey. Patients walked carrying their IV drips, pregnant women gave birth at the roadside and evacuees -- harried by soldiers -- dared not stop to help them.
The bodies of the dead littered the way.
"It was an unbelievable event," Lay Bony, who lost 11 relatives including her husband and children under the hardline communist regime, told AFP.
When the evacuation began, black-clad troops burst into Bony's home, forcing her to leave with her husband, five-year-old daughter and four-year-old son.
"I was afraid that if we did not follow their instructions, we would be killed," said Bony, who had lost a baby during childbirth just weeks before and is even now brought to tears by the memory of the ordeal.
After weeks of walking, the family was first installed at a camp in southern Kandal province and forced into back-breaking work in the paddy fields.
Bony's daughter died within days of their arrival and the harsh living conditions led to her son's death just months later.
Her husband, a soldier from the previous regime, was later arrested and killed, while Bony spent several years in detention where she saw hundreds of inmates taken away for execution.
The 62-year-old recently testified as a witness at Cambodia's UN-backed war crimes court, which is focusing the first stage of a highly complex case against senior regime cadres on the evacuation of Phnom Penh and other cities.
Ex-foreign minister Ieng Sary, "Brother Number Two" Nuon Chea and one-time head of state Khieu Samphan deny charges of war crimes, genocide and crimes against humanity.
The trial has been split to deal with events chronologically, but most observers believe the first "mini-trial" will also be the last due to the worsening health of the octogenarian defendants as well as funding problems.
"We realise that given the advanced age of the accused it is unlikely that they will stand trial for all the alleged mass crimes," said prosecutor Tarik Abdulhak.
Crowds greeted the communist guerrilla group when they first rolled into Phnom Penh on April 17, 1975, hailing the end of a bitter civil war.
But the mood quickly darkened as soldiers fanned out across the city, evicting people from their houses.
"Within a day, the whole nation was turned upside-down by the Khmer Rouge," said Youk Chhang, director of the Documentation Centre of Cambodia (DC-CAM), which researches atrocities committed by the regime.
It was not until 1979 that the Khmer Rouge, led by the late "Brother Number One" Pol Pot, was prised from power by a Vietnamese invasion.
Cambodia's war crimes tribunal has so far achieved just one conviction, sentencing former prison chief Kaing Guek Eav, better known as Duch, to life in jail for overseeing the deaths of some 15,000 people.
Bony, one of almost 1,000 plaintiffs in the evacuation trial, is hoping for a harsh verdict against the former regime leaders.
"I am still suffering," she said. "I miss my husband."