Details have leaked slowly because detainees were ordered not to discuss their experiences with journalists, members of NGOs or diplomats, and many are fearful because of threats to their families.
The accounts have increased concerns among human rights groups about plans to authorise police to hold suspects in secret locations for up to six months without informing their families. Critics say the changes to residential surveillance laws, which would apply in state security, terrorism and major corruption cases, would legitimise forced disappearances.
Scores of people were seized in the clampdown. While some were held only briefly, and several activists and dissidents were formally detained or sent to re-education through labour, others were held for weeks or months at unknown locations in breach of the current law, say experts.
Most signed guarantees they would not write political material online or speak to foreigners. "People are genuinely scared and that's why they are not talking," said Wang Songlian of the Chinese Human Rights Defenders network. "I think all of the people who disappeared for a period of time have experienced some form of torture or mistreatment.
"Even when the [rights] community was being targeted before, people were always quite defiant. This time it is like a bag with a hole punctured: the air has gone out of it."
Relatives have also been silent. But a handful of those held have now disclosed details and the Guardian has spoken to sources with knowledge of others' lengthy detentions. Friends say many of those detained lost significant amounts of weight and show clear signs of trauma, including disturbed sleep and memory loss. Many were hooded before they were driven away and several were badly beaten in the first day or two. In most cases they were watched around the clock by two officers at a time and deprived of sleep.
CHRD, which spoke to the Guangzhou-based rights lawyer Tang Jingling when he was released after more than five months of residential surveillance, said he was questioned non-stop for more than a week by three teams of interrogators, without sleep or rest. He was allowed to sleep for short periods after suffering from trembling, numbness in his hands and chest pains. Another lawyer, Liu Shihui, said on Twitter thathe was interrogated for five days without sleep, until he collapsed.
Detainees were also forced to sit in stress positions for hours at a time. Although it is common practice in Chinese prisons to make people sit still for long periods, it was much more intensely enforced with these prisoners, so even slumping or closing their eyes could prompt a rebuke or threat.
Some were handcuffed for long periods. In other cases, air conditioning was run at full blast until rooms were unbearably cold; one of those thought to have been involved, the lawyer Tang Jitian, has since been diagnosed with tuberculosis, say friends. They are particularly concerned because he has shunned contact with them.
CHRD said another lawyer, Jin Guanghong, was held for days in a psychiatric institution where he was beaten, tied to a bed, given injections of unknown substances and forced to take medicine. He could not recall the full details.
Many found the psychological pressure hardest. They were not allowed to speak or read except in interrogations and guards were permitted to talk to them only when giving orders.
"I tried very hard to adjust my mind from going crazy. There were seven to eight times that I almost broke down. Well, no windows is just a minor problem," wrote Shanghai lawyer Li Tiantian in a tweet after her release from three months of detention.
In an article last week, the artist Ai Weiwei, the best known of the detainees, wrote of solitary confinement: "You truly believe they can do anything to you ... You become like mad."
In some cases, officers threatened detainees with the example of Gao Zhisheng,- a lawyer who has been missing for more than two and a half years and who gave a graphic account of torture when he resurfaced briefly last year. Detainees also told friends of "brainwashing" as officers lambasted them repeatedly for incorrect attitudes and disloyalty to China.
The crackdown appeared to be prompted by anonymous online calls for an Arab-inspired "jasmine revolution", posted on an overseas website. But while most detainees were quizzed about the appeal, several saw it as the trigger rather than the cause of the crackdown.
They were repeatedly interrogated about precise details of contact with other rights activists, trips abroad and meetings with foreigners. Other questions appeared designed to break them down, they felt – for example, asking Li Tiantian about her sex life.
Detainees were also ordered to write and read aloud confessions. Some suspect the resulting videos could be used in future trials, helping to intimidate them into compliance, while others saw it as a way to humiliate activists and sow distrust among them.
"I realise that I face some danger from revealing the truth," Liu Shihui wrote in a tweet.
In a blog post – since deleted – Li Tiantian wrote: "I'll bet that there will be others in the future who, like me, will become increasingly mute, and I now know why many online friends from before have vanished from the internet."
Nicholas Bequelin of Human Rights Watch said the disappearances, and proposed changes to the residential surveillance laws, meant "secret detentions would become the norm for handling dissidents, rights activists and critics. They also tell us that such detentions will be unchallengeable, greatly increase the likelihood of torture and ill-treatment and will be used for political motives far beyond legitimate concerns about national security but rather to protect the party".
The ministry of public security and police in Beijing, where many were seized, did not respond to queries. Nor did the ministry of justice comment on the implications for amending the law.
By Tania Branigan in Beijing