North Korea's political prison camps have grown in size over the past decade and now hold an estimated 200,000 people in "horrific" conditions, Amnesty International said.
A report from the London-based rights group painted a grim picture of one camp in which inmates witnessed public executions and resorted to eating rats to survive.
Amnesty said it had obtained satellite images revealing the location and size of the camps, along with new testimony from former inmates of the Yodok camp complex and others.
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The ex-detainees testified that "prisoners are forced to work in conditions approaching slavery and are frequently subjected to torture and other cruel, inhumane, and degrading treatment", Amnesty said in a report.
All ex-detainees at Yodok in South Hamkyong province had witnessed public executions, it said.
Amnesty said comparison of the latest images with those from 2001 showed a significant increase in the scale of the camps.
"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," said its Asia-Pacific director Sam Zarifi.
Leader Kim Jong-Il is grooming his youngest son Jong-Un as eventual successor.
Amnesty said that in just one camp, Kwanliso 15 at Yodok, thousands are believed to be held after being judged "guilty by association" or simply because one of their relatives has been detained.
Many did not even know what crimes they were accused of.
Amnesty quoted Jeong Kyoungil, a detainee at Yodok from 2000-2003, as saying the working day started at 4am and ended at 8pm but was followed by ideological education.
Only those who finished their assigned tasks would receive the ration of a 200 gram (seven ounce) bowlful of corn gruel, and inmates died daily.
Some prisoners ate rats or picked corn kernels out of animal waste to survive, the report said.
Former detainees have frequently given similar accounts of harsh and life-threatening conditions in the camps. The US State Department in its 2010 human rights report cited estimates of 150,000-200,000 detainees.
