'I was forced into a van and four police officers surrounded me, punching and kicking me. One of them said: 'Beat her to death because she no longer has a son [to settle scores later]'' she said.

Ms Ma said she couldn't remember how many times she had visited Beijing to seek justice after her son, a student at Xian Jiaotong University, was in a traffic accident in July 2000. He died after what she believed was a deliberate delay in helping him.

Ms Ma said petitioners locked in the illegal facilities were subject to routine intimidation and abuse at the hands of the enforcers, many of whom were relatives of officials in charge of petitioning back in their hometowns.

She said the petitioners complained to the community police in Beijing who 'came in and left after a behind-closed-doors chat with hotel operators'.

Ms Ma's plight is shared by thousands on the mainland seeking to have their cases heard.

Wang Jinlan , also from Henan, said she was locked up at the same black jail for two days last month, before activists secured her release. She said they did not know who the enforcers were and what would happen to her and fellow petitioners as they were 'regarded as pigs' by hotel operators and thugs.

Ms Wang has been petitioning against a compensation ruling for a traffic accident since 1995. She said she had lost her land as punishment for embarrassing low-level governments.

She said the system was a joke as it would never honour justice. 'But I have no way back, because I've got nothing to hold to but to keep seeking justice.'