Liu - a recurrent addict - was admitted to Dianbai People's Hospital on June 23 where doctors found he had no vital signs and pronounced him dead.

A day later, police and prosecutors in Maoming and police in Dianbai sent forensic experts to examine Liu's body. They concluded that Liu's stomach had been punctured, that infection had then set in and that he had died after losing consciousness.

'I am a doctor myself and I can say that such a condition is brought about by the application of external force or by gorging on food, but the autopsy only found a small amount of vegetable and some congee in his stomach,' yesterday's Yangcheng Evening News quoted Liu's father, Liu Yihu , as saying.

Liu's parents asked to see footage of the centre's surveillance camera and discovered that a segment between 6.56am and 7.21am on June 22 had been erased.

'We asked why and the centre's director explained that the missing footage was due to a technical failure,' Dr Liu said.

When the recording resumed, Liu was seen wearing chains on his legs.

When contacted yesterday, Dr Liu confirmed the facts as reported and said he was not free to speak to reporters as he was in a meeting with police.

A few hours later, he said: 'Let the matter rest here. I can't speak any more to you because it might affect my safety. I am only looking for justice. I told the Public Security Bureau chief that I agree with his view.'

A staff member at the centre said the director was not available to comment on the case, while a Maoming party committee official said it would be investigated.

Nicholas Becquelin, research director for Human Rights in China's Hong Kong office, said there had been a number of 'irregular deaths' at mainland drug rehabilitation centres, but the authorities had launched a campaign to reduce such incidents.

Mr Becquelin said violence and torture existed in the centres, which were completely closed to non-government organisations and social service providers.

As with the custody and repatriation centre which received Sun - a university graduate who was later beaten to death - inmates are completely under the control of wardens and other staff who exercise strict discipline and used selected inmates as proxies to keep their peers in line.

'Besides, the methods they use are pretty violent in the sense they are forcing people to detoxify,' Mr Becquelin said.

'It is very ineffective because the relapse rate is over 90 per cent.'

The mainland had a serious drug problem that it tried to cover up because it considered it embarrassing, he said.

'It is a hard problem anywhere in the world, but only China has adopted repressive methods.'

Official figures show that the mainland had at least 791,000 drug addicts at the end of last year, with Guangdong accounting for 15 per cent of the total.