One by one they came, taking seats next to a United Nations flag and stating their names for the record. Some kept calm. Some wept. One, as he spoke, used his left hand to clamp his trembling right hand to the table.

They told stories about North Korea’s brutal network of criminal detention and political prison camps, and their evidence was physical: burns on their backs, scars on their heads, bodies ravaged by torture for acts that amount to crimes only in the North. They described forced abortions, public executions, constant hunger and ghoulish mind games played by prison guards, whose permission was needed even to catch and eat rats and mice.

Guards in a good mood would approve, said one defector, Shin Dong Hyuk. Guards wanting a laugh would force prisoners to eat the rodents alive.

Many of the defectors had spoken about their lives before, but this week in Seoul, their stories had a new purpose: as testimony in a U.N. investigation into North Korean rights abuses. Earlier this year, the U.N. human rights chief called those abuses unparalleled and said international attention was “long overdue” — particularly, she said, because they are continuing unabated under North Korea’s third-generation supreme leader, Kim Jong Un.

The three-member commission was established in March and given a year to complete its report. But this weeklong series of public hearings in Seoul, which runs through Saturday, forms the heart of its work, a legal investigation that doubles as horrifying theater.

The investigation, U.N. officials say, could help establish whether the North’s leaders are committing crimes against humanity. But in the shorter term, these hearings, streamed online, are also designed to raise global awareness of a police state that imprisons 150,000 to 200,000 of its people in city-size gulags in the nation’s northern mountains.

According to reports from nongovernmental organizations, the North, at these camps, gives prisoners starvation rations and works them to the brink of death, cutting back the rations further when the work isn’t done well. North Koreans can be imprisoned for criticizing the leadership, watching a foreign-made DVD, leaving dust on the portrait of a leader or attempting to flee the country. Many receive no trial or chance for appeal.

The camps, modeled after Soviet gulags, were established by national founder Kim Il Sung as a way to weed out ideological opponents.

Witnesses on Tuesday and Wednesday said that one could be killed in the camps just for trying to stay alive. Public executions took place semiregularly — maybe twice a year, the witnesses said — probably as a means of keeping other prisoners on edge. One camp survivor, Kim Eun Cheol, said he saw a fellow inmate executed for scavenging a potato from a field. Another was executed for eating herbs.

In both cases, the perpetrators were “almost as good as dead” from torture before their executions, Kim said. “Still, the guards used six to nine bullets to kill them.”

The commissioners mostly asked the witnesses personal questions, in an attempt to draw out their stories. But they would follow up with several clinical questions, apparently seeking to establish evidence about the places and people described.

“Is this the encampment popularly known as Camp 15?” asked Michael Kirby, an Australian judge and chairman of the commission, indicating the satellite image.

“Yes,” said Jeong Kwang Il, a former detainee at the camp.

“And if North Korea were to permit an inspection, you could give us detailed information about this camp?” Kirby asked.

“Yes,” Jeong said. “I can give you exact directions.”

The commissioners have tried to get into North Korea, with no luck. The U.N. Human Rights Council, in documents issued to the media, says its officials have asked the North for “unimpeded access” and cooperation with the investigation. But those requests have so far been ignored.

Shin said his scars were the only physical proof he could show of violations: “But I have to tell you that something is happening there. Massacres are being carried out. We have to talk about it so we can stop it. That’s why I am here.”