More than 100 human rights lawyers and activists have been detained or questioned by Chinese police and denounced in state media as a “criminal gang” in recent days, raising fears of an unprecedented crackdown by the Chinese authorities.

According to human rights groups, a total of 106 lawyers, other staff at legal firms and human rights activists have been detained or questioned and at least three law firms have been searched. Six lawyers from the law firm Fengrui, which has handled a number of high-profile human rights cases, have been detained. Another 17 lawyers and rights activists are missing.

The detentions came as a high-profile Tibetan monk serving a 20-year sentence died in prison and as China was urged to end its two-tier passport system, which restricts freedom of movement for religious and ethnic minorities.

The crackdown began on 9 July when Wang Yu, a Fengrui lawyer, disappeared in the early morning after sending friends a text message saying that the internet connection and electricity had been cut off at her home and that people were trying to break in. Wang’s clients include practitioners of the religious group Falun Gong, which is banned in China.

The firm’s director, Zhou Shifeng, who has also been detained, had represented Zhang Miao, a Chinese journalist who worked with a German magazine to report on the Hong Kong pro-democracy protests and was released last week after nine months in detention.

Li Heping, a well-known rights lawyer who represented the blind lawyer and activist Chen Guangcheng and helped victims of forced evictions, is among those who have not been heard from since being detained.

A large number of the lawyers who have been questioned had signed a public letter condemning Wang’s detention, according to the China Human Rights Lawyers Concern Group, which is based in Hong Kong.

There have been previous government crackdowns on human rights activists and lawyers, including in 2011 during calls for a democratic uprising in the wake of the Arab spring. However, analysts believe this crackdown is unprecedented in terms of its scope. Maya Wang, a researcher with Human Rights Watch, said the detention of activists and lawyers was worrying as it was not in response to “any kind of perceivable threat”.

Articles in state media have denounced the Fengrui lawyers and claimed that they illegally organised paid protests and fabricated rumours online to sway decisions in court. A long article in the People’s Daily newspaper, the mouthpiece of the Communist party, detailed how the lawyers and rights activists gain attention for sensitive cases. It accused them of sensationalising ordinary issues, turning “sensitive issues into political issues”, and not following legal principles.

Wang said this kind of public condemnation by state media was unusual, and that activism itself was being used to justify the detentions. Activism “has basically been deemed illegal by the Chinese government”, she said. “I think it is the most concerning part of the crackdown.”

William Nee, from Amnesty International, said Fengrui’s effectiveness in highlighting cases of injustice worried the government. “We’ve seen cases where public opinion seems to have been mobilised and I think they are worried because they don’t want to lose their grip on public opinion.” Protests outside courts by activists had unnerved the government, he added.

“It is something they have never put up with but especially as it looks like social protests are on the rise, strikes are on the rise, there is the potential for economic uncertainty. I think all these factors have together in the government’s mind made them want to crack down on human rights lawyers.”

The US State Department condemned the detentions and said it was concerned that the new national security law was being used as a “facade to commit human rights abuses”. It called on China to “respect the rights of all its citizens and to release all those who have recently been detained for seeking to protect the rights of Chinese citizens”.

Meanwhile, Human Rights Watch (HRW) has called on China to ends its use of a two-tier passport system. Under the system, residents from areas that have large Tibetan and Muslim populations have to provide more extensive documentation than other Chinese citizens.

According to HRW, there is a fast-track passport application process for residents in areas populated by the majority Han Chinese that is denied to people in areas populated predominately by Tibetans and Muslims. An HRW report identified cases where members of religious minorities faced delays of five years in getting a passport or were refused one.

“The restrictions also violate freedom of belief by denying or limiting religious minorities’ ability to participate in pilgrimages outside China,” said Sophie Richardson, the China director at HRW. Extra restrictions in Tibet since 2012 have stopped most residents travelling abroad, while attending any events in other countries, such as teachings by the Dalai Lama, the exiled spiritual leader of the Tibetan people, is considered to be subversive political activity.

A Tibetan lama, Tenzin Delek Rinpoche, has died in prison, 13 years into a 20-year sentence for what human rights groups say were false charges that he was involved in a park bombing. He was 65. The cause of death was not clear, but according to a statement by the group Students for a Free Tibet, he had been suffering from serious health problems and had been refused medical parole.

Tenzin Delek Rinpoche was arrested in 2002 for alleged involvement in a bomb attack in Chengu, the capital of Sichuan province, and was initially sentenced to death. His sentence was later suspended and changed to life imprisonment.

Tenzin Dolkar, the executive director of Students for a Free Tibet, said: “His death is a harsh reminder of the violent and brutal reality of Chinese-occupied Tibet.”