The mainland's internet police have struck again, closing down or suspending several popular microblogs in the past week as they step up a crackdown on online discussion of politically sensitive issues. One well-known blogger, Wang Xiaoshan, said friends told him yesterday morning that his Sina.com microblog had been closed down. He said he had no idea what was behind the closure or what he had done wrong. Wang said he had written more than 36,000 postings and his microblog had attracted 570,000 followers. He said he had also received a notice from Sina.com management yesterday, but could not disclose its contents. 'But I can tell you one thing for sure, the closure was not carried out by Sina.com voluntarily,' he said. 'But I am not shocked by the decision, given that anything can happen in this country.' Mainland censors introduced a real-name registration requirement for microbloggers in March, a move critics said would enable tighter censorship. The authorities, determined to maintain social stability, have grown increasingly wary of online discussion of political and social issues. A number of other popular bloggers have run foul of the mainland's censors in recent weeks, as authorities have scrambled to contain the fallout from the downfall of influential Chongqing party secretary Bo Xilai and blind activist Chen Guangcheng's flight to the US embassy in Beijing after escaping his illegal house arrest. Wang said he had no idea which postings on his microblog had resulted in its closure, but did admit to reposting an item discussing the rumoured resignation of Premier Wen Jiabao . The microblogs of a number of vocal bloggers, including Professor He Bing from the China University of Political Science and Law and journalists Yang Haipeng and Shen Yachuan , have been closed recently. Hao Qun, a novelist-turned-blogger who uses the pen-name Murong Xuecun , said his microblog, which had 1.85 million followers, was suspended on Thursday and he had learned it could last a month or so, until after the 23rd anniversary of the bloody crackdown on the pro-democracy movement in Tiananmen Square on June 4, 1989. He said he had also learned that the order to suspend his microblog had come from the government's top internet censor. Hao said he imagined that the suspension was punishment for his comments on the ongoing Chen saga in the overseas press and his attempt to visit Chen in Shandong in October. 'If you'd asked me or any other bloggers, we'd have all told you with confidence that we knew where to draw a line, but apparently we're all wrong,' he said. 'The order could just come from anyone at the top or even his secretary, who simply call the censors because they bump into a posting they're not happy with.'