'If you don't answer, we'll strip you naked and put your photos on the internet," rights activist Liu Shasha quoted one officer as saying after Beijing police kidnapped her last July.

Police officers had already pressed Liu to the floor, hit her, trodden on her head, badly twisted her arm, drenched her in cold water and gagged her with a cloth soaked in spicy chilli oil, she said.

They eventually gave up trying to torture her into giving information about fellow activists and dumped her on a remote road the next day. As Liu was recovering from her ordeal, she received a surprise visitor: internationally acclaimed artist Ai Weiwei. Ai, 53, had seen online news of Liu’s kidnapping and decided to comfort her. The artist has become a figurehead of China’s Twitter-using network of rights activists in recent years and has investigated some of the many alleged abuses by police and officials. He is no stranger to these himself.

Ai and his studio staff documented the more than 5000 schoolchildren who were among 87,000 dead and missing in the 2008 Wenchuan earthquake in Sichuan province. They highlighted the poor construction of some school buildings that collapsed in the earthquake and supported local activists such as Tan Zuoren, who was sentenced to five years in prison for subversion.

During Tan’s trial in August 2009, plain clothes police attacked Ai and other activists in their hotel room in the provincial capital, Chengdu. Ai later underwent emergency surgery for a cerebral haemorrhage caused by the beating.

Following his detention at Beijing’s main airport a week ago, the ruling Communist Party has said only that Ai is under investigation for unspecified “economic crimes”. His wife, Lu Qing, has appealed to Beijing police to tell his family the reason for his detention, where he is held and what legal procedures are in process.

“We feel that the government has been defying its own laws in handling Ai Weiwei’s case,” said Patrick Poon of the Hong Kong-based China Human Rights Lawyers Concern Group.

Many other prominent dissidents have disappeared since mid-February, while police have temporarily detained, harassed or placed under house arrest scores of others after anonymous online calls for peaceful “Jasmine” rallies against the government. Organisers called for “strolling” protests in urban centres on Sunday afternoons, adopting a similar tactic to East German and Polish democracy activists in the 1980s.

No large rallies have been reported but several rights activists said they “strolled” at designated sites in Beijing, Shanghai and the northern city of Tianjin. An estimated 1000 protesters gathered in Shanghai on February 27 but numbers apparently dwindled in the following weeks. Police have tried to prevent reporting from the protest sites by foreign journalists.

“We feel very sad that the government is just trying to crack down on high-profile dissidents as a way of warning other dissidents that they can do anything they want,” Poon said. “The government is now so nervous about the online calls for Jasmine revolution.”

Some activists were released but kept under police surveillance and warned not to take part in protests or other activities. Police have held Gu Chuan, a dissident writer and blogger, for six weeks without any legal procedure in northern Beijing, his wife said. Several of China’s best-known rights lawyers disappeared around the same time as Gu.

The UN Working Group on Enforced or Involuntary Disappearances on Friday issued a statement of “serious concern at the recent wave of enforced disappearances” in China.

“Enforced disappearance is a crime under international law,” the UN group said. “There can never be an excuse to disappear people, especially when those persons are peacefully expressing their dissent.”

Teng Biao, an outspoken signatory of the Charter 08 for democratic reform, was one of the lawyers mentioned in the UN statement.

“I don’t need it. I can just do it,” a uniformed police officer told Teng on February 18 when the lawyer questioned the legal basis of the officer’s demand to check his identity card. “I believe you are being obstructive. If you don’t co-operate with our work, the consequences will be on your own head,” the officer told him, according to video footage of their heated discussion.

Teng, who is also a law professor, had travelled to the Beijing home of another activist. He told police they had no legal right to require every visitor to register. Nothing has been heard of Teng since he disappeared the day after.

Poon said rights lawyers such as Teng were operating in “a very difficult and dangerous situation”. Among the recent cases pursued by Teng and colleagues was the death sentence against Leng Guoquan, a man convicted of smuggling drugs from North Korea after a confession he said was extracted by severe torture. The pursuit of Leng’s case in the northeastern city of Dandong could be part of the reason for Teng’s detention, Poon said.

One of Ai’s assistants travelled with Teng in December to Dandong to make a video about Leng’s case. Ai’s team has documented other controversial legal cases, including the death of village head and land-rights activist Qian Yunhui in Zhejiang province on December 25.

Local authorities insisted that Qian died in a traffic accident, but his death provoked an outcry from internet users and rights activists. Some claimed witnesses had reported seeing unidentified people push or throw Qian under a truck, and police fought scores of villagers who staged protests after the popular leader’s death.

“Why didn’t the local police release a forensic assessment report?,” Ai said in an interview with state media in January. “Why would a troublemaker accidentally die under the wheel of a truck? Why did police arrive so soon to handle the emergency?” he asked. “All these factors point to Qian’s predestined death: a fact that has ruined our faith in justice.”

Just five weeks after Qian’s death, a local court sentenced the unlicensed driver of the truck to three-and-a-half years in prison and the government tried to quell public debate over the case.

 

In response to the international outcry over Ai’s detention, foreign ministry spokesman Hong Lei said on Thursday Ai was subject to a criminal investigation that had “nothing to do with human rights and freedom of speech”. Hong again said China was “a country ruled by law”. “Other countries have no right to interfere,” he said.

German foreign minister Guido Westerwelle had summoned the Chinese ambassador to Berlin to protest about Ai’s arrest. British Foreign Secretary William Hague said he was “very concerned” by the arrest and called for Ai’s release. He said the government “remains committed to engagement with China on human rights”.

Many international artists, scholars and writers have also voiced support for Ai, whose Sunflower Seeds exhibition runs at Tate Modern on London until May 2.

Poon said his lawyers’ group planned to join a protest today outside the Chinese government’s liaison office in Hong Kong, which enjoys far more freedom than mainland China under the principle of “one country, two systems”. He said the group would highlight the detention of Ai, the rights lawyers and other activists.

Ai has said he believed the time was not ripe for revolutionary change in China. “It only needed 18 days for the collapse of an apparently harmonious and stable military regime that lasted 30 years,” he said on Twitter in February about the fall of former president Hosni Mubarak’s regime in Egypt. “This 60-odd-year thing might need a few months,” he said in a sarcastic posting, referring to the Communist Party’s rule since 1949.

Ai’s reputation has grown with a wide range of artworks, installations and performances. He was an artistic consultant for the Bird’s Nest Olympic Stadium in Beijing. Strong-willed and sometimes anarchic, some of his recent work has straddled the worlds of art and activism.

Almost two decades after he returned to China in 1993, following 12 years in the United States, he has found himself increasingly at odds with the social control and censorship in his homeland. Ai said he cancelled a solo exhibition planned for a leading Beijing gallery this year after the organisers requested a delay because of the “political sensitivity” of his work. He quoted a local official as saying the order to demolish his Shanghai studio in January had come from the central government.

Two weeks ago, he said his plan to open a studio in Berlin this summer was partly motivated by growing interference in his work, but he hoped to continue spending most of his time in China.

“However, there will be no choice if my work and life are somehow threatened,” he said.