Shortly after 4am she sent another message, saying someone was forcing her front door open. That was her last message before she disappeared. Her friend tried to call her back later, but there was no answer.

No one has been able to contact Wang or her husband since then. Friends who had her house keys tried to get into her home last Friday, only to find the locks had been changed. They could not find any record of her husband and son's departure from the airport. A security guard at her housing compound told them he saw someone being taken away by dozens of policemen in the early hours of last Thursday.

The incident was only the start of an unprecedented crackdown on mainland human rights lawyers over the past week, which so far has seen about 215 lawyers and rights advocates taken away, summoned or detained by police, according to figures compiled by China Human Rights Lawyers Concern Group. The offices of at least three law firms have been searched.

Although most have since been released, at least 14 people are feared still detained by police. And another 11 have been placed under criminal detention or "residential surveillance" either on unspecified charges, or "incitement to subvert state power" and "seeking quarrels and provoking trouble".

Veteran China watchers say the police action is the worst since the 1989 Tiananmen crackdown. They also say it shows the mainland authorities' fear of the fast-growing civil society and their wariness over the crucial role played by an expanding community of rights lawyers in the grass-roots "rights defence" movement.

Analysts say that these rights lawyers - whose number has grown at least tenfold to 200-300 from a decade ago - have long been seen as a thorn in the side of the authorities for using litigation and advocacy to help members of the public assert their rights and seek justice under the Communist Party-controlled legal system.

But while the crackdown was nationwide, it appeared to have a specific target: the partners and lawyers at one Beijing law firm - Beijing Fengrui, where Wang works.

Fengrui has a reputation for handling prominent rights cases, including artist Ai Weiwei's 81-day detention in 2011. It describes itself on its website as "full of passion in our pursuit of liberty, equality and democracy".

The day after Wang disappeared, several lawyers and staff at the firm, including Zhou Shifeng, the director of Fengrui, his assistant Liu Sixin, and lawyers Li Zhuyun, Wang Quanzhang and Huang Liqun were also either taken away or disappeared. Several other people at the firm had also been taken away.

On Sunday, party mouthpiece went so far as to depict Wang Yu and her husband, Bao Longjun, Zhou, Liu, Huang, who were the first to be taken away in the crackdown, as "a major criminal gang" that "seriously disturbed social order". It said they had been placed in criminal detention for "seriously violating the law", but did not name a charge.

Analysts say the crackdown is an effort by the authorities to discredit the rights defence movement and sully the reputation of the lawyers and its advocates. They also say it is also meant as a warning to other lawyers not to take on rights cases.

Eva Pils, a China legal expert at King's College, University of London, said that in recent years, mainland rights lawyers had formed themselves into a community, often coordinating their actions to support colleagues assaulted while working on rights abuse cases. "Since so many lawyers started openly identifying with human rights causes and coordinating their advocacy campaigns, they are one of the closest things China has to a political opposition," she said.

This was the case with the fatal shooting in Qingan, in Heilongjiang province in May of an unarmed man. Xu Chunhe was shot dead by a policeman at a train station in front of his mother and children, leading to a public outcry over concerns the policeman used excessive force.

said the public outrage over the Qingan case was the result of lawyers "masterminding plots" and "colluding" with activists and petitioners to create public disorder "in the name of 'rights defence, justice and the public interest'".

After the shooting, lawyer Xie Yang, who represented Xu's mother, was attacked by a gang of unidentified men. Several lawyers also travelled to Qingan but were detained. Their detentions triggered public outrage and hundreds of fellow lawyers issued a joint statement condemning police abuse. This pattern of support has been repeated over many incidents - and lawyers say the authorities now want to put a stop to this.

"What rattles [the authorities] is that civic-minded lawyers have the capacity to get together in such large numbers to protest [against] what they see as illegal behaviour on the part of the state," Pils said. "They want to break this model of coordinated, vocal defence of human rights lawyers' right to defend rights."

The coverage on Sunday labelled the lawyers as the "black hands", or masterminds, behind rights defence activities, accusing them of "confronting the court" and getting "troublemakers" to rally around sensitive cases they wanted to publicise.

It also accused Wu Gan, a flamboyant campaigner better known by his nickname "Super Vulgar Butcher", of plotting with rights lawyers to draw public attention to more than 40 "sensitive cases", including the Qingan shooting.

Wu, who also worked at Fengrui, was charged this month with "inciting subversion" and "provoking trouble".

As in many past cases where lawyers were attacked or detained, Wang Yu's disappearance prompted her colleagues to immediately express their solidarity. More than 110 lawyers issued a joint statement last Friday to condemn her detention and voice support for her. By late last Friday and early Saturday, many of those who signed the statement were detained one after another across the country. Those released said they had been warned against voicing support for Wang Yu and Fengrui staff.

On the mainland, because the laws meant to protect citizens' rights are often flouted by government departments and officials, and outcomes in the party-controlled courts are often predetermined, lawyers and human rights advocates often have to resort to social media campaigns and protests to promote their cases and raise public awareness about the victims' ordeal.

"[They] have to ensure that laws and regulations are actually adhered to: not just to give their clients justice and to hold the government accountable, but they have also used creative advocacy tools to enhance the social impact of their cases." said William Nee, China researcher at Amnesty International.

Teng Biao, a fellow at Harvard Law School, said the authorities were nervous about the large numbers of rights lawyers, who used the law to challenge the government and officials' abuse of power. "They fear the lawyers will unite into a force to pose a challenge to the current political system," he said.

Teng said that after the national security law was passed this month, law enforcers and security departments now also had more justification to suppress activities perceived to be a threat to national security.

The sweeping action against lawyers came amid a tightening of ideology overseen by the administration of President Xi Jinping , said political commentator Ching Cheong.

Ching, a China watcher for nearly 40 years, said the unprecedented crackdown on rights lawyers was in line with Xi's conservative ideology, which regards human rights advocates and government critics as a threat to the regime.

He said the rhetorical foundation of the crackdown could be traced to a 2012 article in , which cautioned that the United States would use five categories of people - rights lawyers, underground religious followers, dissidents, opinion leaders on the internet and the underprivileged - to "infiltrate" Chinese society to push for a regime change.

Nee said the central government was pushing an "irreconcilable set of goals", by wanting to promote the rule of law, but at the same time suppressing lawyers who held officials accountable.

"With this crackdown, the government is trying to send a clear signal to the rights lawyers: no taking on 'sensitive' cases, no social media advocacy, and no street protests," Nee said.

"With strikes and other social protests growing in scale, and with the possibility of decreased economic growth ... there's no doubt that 'maintaining social stability' will be the government's top priority."

Poignantly, just days before her own detention, Wang Yu spoke against the incrimination of citizens for expressing their opinions.

As Wu Gan's former lawyer, she argued he should have the freedom to criticise the government. "As citizens, people should be able to question the government without being accused of 'inciting subversion'," she said. "This is political persecution."