Chinese authorities have detained prominent human rights lawyer Xu Zhiyong. Xu was taken away by police this week as part of China's clampdown on dissent ahead of the politically sensitive 60th anniversary of the founding of Communist Party rule.

Members of Xu's civil rights organization, the Open Constitution Initiative, or Gongmeng, as it is called in Chinese, say they learned their colleague had been taken away late Thursday.

Teng Biao, one of Gongmeng's founders, says Xu was supposed to appear at a hearing Thursday, but never showed.

Teng says at that point it had already been two days since his colleagues had heard from Xu, so they went to his home to see if he was there. Teng says a security guard at the housing complex where Xu lives says he was taken away by police early Wednesday morning.

Gongmeng, a group of lawyers and scholars, has helped victims of China's tainted-milk scandal and people with human rights issues. It is a vocal critic of government policies.

Neither members of Xu's group nor his family have heard from him since he was taken away. According to the group's Website, Xu is the second member of Gongmeng to go missing this week.

Teng says that as the Chinese government prepares to mark the 60th anniversary of Communist Party rule in October, the situation is tense. He says Gongmeng is not the only one being targeted.

Teng says that in addition to Gongmeng, many other human rights lawyers have not received renewal of their licenses and others have recently had their licenses revoked. Yirenping - an organization that focuses on the rights of hepatitis patients - has also faced a clampdown.

Earlier this week, Chinese authorities seized dozens of Yirenping's newsletters and searched its offices. The group says authorities are accusing it of illegal publishing, and it denies doing anything wrong.

Wang Songlian, research coordinator at the pressure group Chinese Human Rights Defenders, says the recent unrest in northwestern Xinjiang appears to have made officials jittery.

"The riots in Xinjiang probably had an impact on authorities' level of anxiety as the anniversary approaches as well," said Wang. "And so that might have made authorities much more aggressive toward these organizations. And we are worried that other organizations that promote human rights and civil rights and human rights defenders in general are going to suffer more as the anniversary comes."

The government began targeting Gongmeng in mid-July when tax authorities issued a hefty fine against the group, accusing it of tax evasion. Authorities also raided its office in Beijing and formally banned the organization. Xu, a co-founder and the head of the group, was supposed to appear at a hearing Thursday to defend his organization from the charges.