The latest death, of Hubei resident Li Wenyan, 50, in Jiujiang, Jiangxi province, is the fifth case linked to custodial abuse to surface in the past two months.

Jiujiang officials attributed Li's sudden death to fright from a 'dreadful nightmare' and he was later confirmed dead at hospital last Friday, the People's Daily website reported.

But Li's family said they believed he was beaten to death by prison bullies because a few bruises were found on his forehead and the officials in charge of his custody could not give a convincing explanation.

Custody records showed Li had five physical examinations since being locked in a 14-person cell for stealing 1,500 metres of electrical cable with accomplices in January. Health reports suggested Li was generally in good health, albeit a little 'feeble'.

Li's family said it was strange a healthy man would suddenly die because of a nightmare and requested that prosecutors order an autopsy.

In February, Yunnan authorities also initially said detainee Li Qiaoming had died accidentally while blindfolded and playing a game of hide-and-seek with other inmates, but a later investigation revealed the beating by bullies.

Also, one inmate each in Hunan, Shaanxi and Hainan provinces has died behind bars at the hands of prison bullies. Hainan inmate Luo Jingbo, 58, was beaten to death last month after he refused to shower.

Mainland detention centres are usually run by underpaid police officers who are notorious for using torture to obtain confessions. There are no signs that the continued exposure of prison abuse cases has improved conditions for inmates.

The Public Security Ministry said on Tuesday that police across the mainland would be re-educated, assessed and urged to eliminate abuse of authority and abuse by inmates.