Kim Hye-sook reaches for a wooden coffee stirrer on the table in front of her to illustrate how public executions were carried out in the North Korean labour camp that was her home for nearly three decades.

"A day before the executions, prison guards would put huge banners to tell everyone what was going to happen, and on the day everyone would be ordered to attend," the diminutive 50-year-old explains. "They would take the prisoner to a stake, tie them up and blindfold them. The firing squad would let off 30 or 40 shots until the prisoner's body had turned to honeycomb. Every time the bullets hit, the stake would crack backwards."

Obtaining testimony about North Korea's gulag system is notoriously difficult. Once inside a labour camp few political prisoners are ever granted their freedom and even fewer ever make it over the border to describe their ordeal to the outside world. Mrs Kim is one of the most recent defectors to find safety and – in her first interview with a Western newspaper – she describes a penal system that is shocking in its outright barbarity as North Korea continues to defy the international community with a human rights record that echoes the worst excesses of Stalin's Soviet Union.

Mrs Kim's only crime was what Kim Jong-il's regime calls yeon-jwa-je – guilt by association. In the early 1970s her grandfather defected to South Korea and under North Korea's system of collective punishment for political crimes, the entire family was rounded up. "We were living in Pyongyang," she explains, referring to North Korea's capital. "I was just 13 at the time and the whole family had been classified by the state as a 'dangerous element'."

Ordered to leave her home by armed guards, she would not see the outside of a labour camp for the next 28 years. Mrs Kim was taken to Bukchang, a gulag run directly by the interior ministry, which refers to it by its bland official title: Kwan-li-so (penal-labour colony) No 18. A sprawling complex that straddles the Taedong river, it houses an estimated 10,000 inmates, the vast majority of whom are political prisoners serving life sentences in a country where life really does mean life.

The regime is slightly less strict than the camps at Yodok and Kaechon, but beatings, starvation and summary executions are still common. "We were always hungry," recalls Mrs Kim. "Every day was a struggle to find food. The camp provided a single meal of corn gruel, but it was never enough. We would go out looking for anything green to eat. The most popular item was acorn leaves as they were easier to digest." The misery of malnutrition was compounded by long bouts of forced labour – the average working day was 16 hours. The "lucky ones" worked on farms or in the prison itself but most toiled in coal mines that fed the nearby power station, slowly succumbing to exhaustion and disease.

As the decades passed, Mrs Kim's grandmother died after years of hunger and her mother and brother were killed in work accidents.

According to Kim Joo-il, a former army officer who defected in 2005 and now lives in Britain, the unrelenting mercilessness with which political inmates are treated is drummed into the country's elite, who have the task of protecting the state's Stalinist ideology at all costs. "People in labour camps are not looked upon as humans," he explains. "They are enemies of the state who have no rights whatsoever. You could kill them with your own hands and nothing would happen to you."

Freedom for Mrs Kim came in 2001 in an amnesty for political prisoners to celebrate Kim Jong-il's birthday. But it would not be the last time she saw Bukchang. With the help of friends, she crossed the border into China. North Korean women are particularly vulnerable in China. Many Han Chinese treat ethnic Koreans with contempt and sexual trafficking is rife. "I crossed over with a 23- and a 27-year-old," Mrs Kim remembers. "They were sold for 30,000 yuan (£2,900)."

Mrs Kim, who was 43, was saved by her comparative age and found work in a Korean barbecue restaurant. But even though she had escaped North Korea, her boss asked her to return to buy piglets which, thanks to the country's skewed economy, were around a tenth of the price of pigs sold in China. As an illegal immigrant she felt she had no choice but to agree.

The North Korean government views any attempt to leave the country without permission as political dissent. Mrs Kim lasted two weeks before she was found and returned to Bukchang in the summer of 2008. Held in a less restrictive corner of the camp while the authorities decided what to do with her, she managed to escape and returned once more to China. From there she made her way overland, via Laos and Thailand, to South Korea, where she now lives.

Despite growing testimony from the few inmates that make it to the West, the North Korean government refuses to acknowledge the existence of labour camps. But satellite photos show at least six camps across the country, housing an estimated 200,000 people. Amnesty International recently published images of the six camps and reported significant expansion across the country.

"These are places out of sight of the rest of the world, where almost the entire range of human rights protections that international law has tried to set up for the last 60 years are ignored," says Sam Zarifi, Amnesty's Asia Pacific director. "As North Korea seems to be moving towards a new leader in Kim Jong-un and a period of political instability, the big worry is that the prison camps appear to be growing in size."