At least three activists campaigning for senior Communist Party officials to disclose their family assets were in custody yesterday for staging an illegal gathering, Liang Xiaojun, the lawyer for one of the detainees, said. Liang said his client, Yuan Dong, was taken into police custody along with three other activists for criminal investigation after they held banners in the Xidan Culture Square in Beijing's Xicheng district on Sunday afternoon calling for greater disclosure of officials' assets to improve the transparency of governance and combat official corruption. Yuan, 45, was being held in the Beijing No3 Detention Centre along with two other activists, Zhang Baocheng and Ma Xinli, Liang said, but the whereabouts of the fourth activist, Hou Xin, was unknown. Beijing police did not respond to an inquiry about the detentions yesterday. The activists' street campaign is part of a broader civil campaign launched in December calling for top government officials to declare their assets publicly to help fight rampant official corruption. Thousands of mainlanders from a wide range of social groups have signed an open letter calling for the members of the Communist Party's new Central Committee to disclose their family assets, saying the initiative would be the most effective way to rein in corruption. The letter, sent to the National People's Congress, argues that the 205 members of the Central Committee are the most powerful people on the mainland and that by setting an example they could make a difference in the fight against corruption. Sun Hanhui, one of the co-ordinators of the online campaign, said it had attracted 7,673 signatories by yesterday afternoon. We'll continue pushing it forward until the day when we see legislation for disclosure of officials' wealth Sun Hanhui "We'll continue pushing it forward until the day when we see legislation for disclosure of officials' wealth," he said. However, the authorities have grown increasingly wary of the social implications of the massive mobilisation and intensified a crackdown on such activism. Zhang Kun , an activist from Xuzhou, Jiangsu province, who went on a nationwide tour to promote the campaign with another activist earlier this year, was detained for 40 hours by state security police in Beijing on March 2, a day before the annual session of the Chinese People's Political Consultative Conference opened in Beijing. "They told me that public declaration of officials' assets is at odds with the country's current situation and we should stop doing it, as our campaign would lead to social instability," he said. Zhang was later intercepted in Tianjin by Xuzhou police and sent back to his hometown. Several other prominent campaigners, including activist Xu Zhiyong, lawyer Ding Jiaxi and Sun himself, have been warned about their involvement in the campaign on several occasions. Yuan, who was detained on Sunday, had his home searched by Beijing police last month after campaigning in front of a Carrefour store in Beijing's Zhongguancun area, his wife said. Liang, Yuan's lawyer, said his client was in good spirits but could be locked up for months before being charged.