North Korean defectors have given international investigators harrowing accounts of brutality and abuse in the country's prisons.
A UN panel has begun interviewing witnesses in the South Korean capital, Seoul, and will move onto Japan.
One of the first witnesses to testify spoke of how he had been forced to watch the execution of his mother and brother, after they had attempted to escape a camp.
He said guards also chopped off one of his fingers after he dropped a sewing machine.
Another former inmate described how a woman had been forced to kill her own baby.
The BBC reports the hearings will last until Saturday, and more than 30 witnesses are expected to testify.
The government in Pyong Yang refuses to recognise the commission and is denying its investigators access to North Korean prisons.
The panel is due to present its report to the UN Human Rights Council in March 2014.