The men from China's national security brigades came for Natalie Qiao's parents at 10pm on 8 June. Five young men in plain clothes bundled the elderly couple into an unmarked car.

The crime of Qiao Yongfang and his wife, Yan Dongfei, both aged 60 and residents of Huhot City, in Inner Mongolia, was to be practitioners of the banned Falun Gong religion, which has tens of millions of followers in China.

Ten years after the prohibition on Falun Gong was ordered by China's former leader Jiang Zemin, commencing a brutal crackdown on its adherents, believers such as Qiao's parents are still being pursued, despite international protests.

The only change is that the persecution is now more secretive.

Amid new protests in Britain, including a march today from the Chinese embassy to Chinatown to mark the anniversary of the Falun Gong crackdown, Natalie Qiao and a former detainee have described the continuing abuse, providing a detailed insight into how those deemed enemies of the Chinese state are dealt with.

Anyone practising religious observance outside officially sanctioned channels, including members of unofficial Catholic churches or Protestant house churches, risk detention. Other groups at risk include Muslims in the Xinjiang Uighur autonomous region, especially those branded as religious extremists by the authorities.

Falun Gong has been treated most harshly. Supporters outside China claim 2,000 have died in custody since 1999, and the UN alleges that the group's members feature disproportionately among those who have suffered torture and abuse. It is this that Qiao – who will hand in a petition to Downing Street on Monday – fears most.

"They took my parents after dark. They don't want people to know. The persecution is almost underground," said the 35-year old purchaser, who lives in Watford. "I got a call from one of the members of my family in China. They had tried to call my parents' phone and a strange man had answered and demanded to know who was calling. They do that to find other members of Falun Gong.

"My uncle then went to my parents' house. The building attendant didn't want to talk but finally he said what happened. But even then the local police would not admit they had been arrested. They said they didn't know. In the end we heard it through a friend in the police who told us they had been taken to detention centre number one.

"We are not allowed to talk to them. When we rang the National Security Brigades they said my parents were not co-operating. They have not written a letter denouncing Falun Gong or given names of other practitioners. My extended family at first refused to believe what was going on. They said the persecution of the Falun Gong was over. But it's happening every day."

Natalie is terrified about what is happening to her parents, amid well-attested reports of the serious maltreatment of Falun Gong detainees who refuse to renounce their beliefs, as well as darker, unproven allegations that some who have died in custody have had their organs harvested.

Annie Yang, an antiques dealer who fled to London after being released from a re-education through labour camp, knows at first hand what Qiao's parents are going through.

"I was arrested in March 2005," she said. "I was living in Beijing and was a practitioner of Falun Gong. They came for me in the evening. I'm a single mother and I was with my 16-year old son. Only one of the men was wearing a uniform. None of them showed ID.

"I told them I was a mother. But they took me and left my son on his own. They took me to a detention centre where they kept me for 40 days without access to a lawyer. At the end they said I had been sentenced to two years in a labour camp for being a member of Falun Gong."

According to Yang, it was commonplace until 2004 to use physical violence to make members recant and give up more names. She was subjected to a more insidious abuse.

"The camp made gloves. But I was not allowed to work or have enough food or water until I renounced my beliefs. I was made to sit on a stool for 21 to 22 hours a day. I had to keep my back straight and my knees and feet pressed together with my hands flat on my thighs. I was told I was not allowed to close my eyes. If I did, they would swear at me.

"If I wanted a drink I had to say: 'Please class leader...' and before I put down my cup: 'Please class leader...'. The only food I was allowed was half a 30g Chinese bun. It was hard and sour. No vegetables. I became so thin. It was so hot too. In the 40s. And they would not give me enough water.

"After three months I could not take it. I was nearly mad. I renounced. They force you to. They say if you don't we will extend your sentence. Against my conscience I gave them names too."

The persecution of Falun Gong is all the more peculiar for the innocuousness of the religion. Without any real formal leadership structure and no role of membership as such, adherents of the religion, founded by Li Hongzhi in 1992, follow so-called traditional qigong practices for both spiritual and physical development as detailed in Falun Gong's literature. Without formal rituals of worship, its central tenets are truthfulness, forbearance and compassion.

Falun Gong, which emerged in large part out of ideas prevalent in some aspects of alternative Chinese medicine, came to be regarded as a threat to the Chinese state after 10,000 of its practitioners staged a silent protest at the Communist party's headquarters in April 1999 to complain about attacks on its members, a move that led to its banning two months later. Accused of being an unregistered religion, spreading superstition and defrauding people, official organisations – crucially – attempted to suggest it was a politically motivated organisation, suggesting the real motive for the crackdown: the perceived threat it posed to the Communist party by its massive appeal.

Amnesty International's UK director, Kate Allen, who has been following Qiao's case, said: "This is a heartbreaking story – Natalie's parents were due to visit the UK to see their grandchildren at the end of the month. Now instead of preparing for a family visit, Natalie is worrying about their safety in a Chinese detention centre.

"Nobody should be locked up for their peaceful religious beliefs. Yan Dongfei and Qiao Yongfang should be released immediately and unconditionally, unless the Chinese authorities are going to charge them with an internationally recognised crime and give them a fair trial."