International rights groups called on Wednesday for the release of dozens of activists being held across China for showing support for Occupy Central's mass pro-democracy protests in Hong Kong. At least 20 people have been detained by police in a number of mainland Chinese cities after they posted photographs of themselves with shaved heads as a message of support for the protests, which are calling for genuine universal suffrage in 2017 elections in the former British colony. At least 60 more have been called in by state security police for questioning, Amnesty International said in a statement on its website. "The Chinese authorities must immediately and unconditionally release all those detained for peacefully showing support for pro-democracy protests in Hong Kong," the group said. The ruling Chinese Communist Party's censorship machine, known colloquially as the Great Firewall, has blocked and filtered keywords linked to the pro-democracy movement in Hong Kong and banned the photo-sharing service Instagram in a bid to keep its more than 600 million netizens in the dark about developments there. Among those detained or held under guard at their homes to prevent them from traveling to Hong Kong to join the demonstrations were Hunan activist Ou Biaofeng and Shenzhen-based Wang Long, detained on criminal charges of "picking quarrels and stirring up trouble," it said. In the southern city of Guangzhou, police seized dozens of activists and citizens who gathered in the Martyr Memorial Gardens to show support for the Hong Kong protests on Tuesday, the overseas-based Chinese Human Rights Defenders (CHRD) group said in an e-mailed statement. The group, which monitors and collates reports from rights groups inside China, said an estimated 20 people were detained and taken to unknown locations. Luo Xiaoxiang, another activist from Guangzhou, capital of Guangdong province which borders Hong Kong, was also detained, while Xie Dan and Luo Yaling are being held in the southwestern megacity of Chongqing, Amnesty said. Shanghai-based Chen Jianfang and Shen Yanqiu were also named, along with Song Ningsheng, Gong Xinsheng and Chen Maosen from the eastern province of Jiangsu. In Beijing, police have imposed tight domestic surveillance on Liu Huizhen, Li Dongmei, Guo Zhiying, Chen Lianhe, Wu Xiaoping, Han Shuzhen, Cui Baodi and Zhang Chonggang, Amnesty reported. Circumvention software While the complex system of filters, blocks and human censorship severely limit what Chinese netizens are able to see online, activists and intellectuals are increasingly making use of circumvention software and virtual private network (VPN) services to "scale the Wall" and read blocked content. The result has been a slow filtering of news back into China's civil rights and activist community, and a growing wave of support for Hong Kong's bid for full democracy. Online activist Wu Bin, known by his online nickname Xiucai Jianghu, said accounts on popular social media platforms in China are consequently being close left and right. "A lot of accounts are being shut down; it's really serious," Wu said. "I shaved my head in support of Hong Kong, took a photo and posted it online." He said the Twitter-like service Sina Weibo had immediately shut down his account. "A lot of my friends have had their accounts closed as well," Wu said. "It's much worse than it used to be." Messaging app In the eastern province of Shandong, civil rights campaigner Li Shufen said she had learned about the Hong Kong protests via the messaging app WeChat, which is hugely popular in mainland China. "Everyone is very supportive of the Occupy Central campaign for democracy in Hong Kong," Li said. She added: "They don't really report it. All the news here is the government's point of view." "We have to go online to read about it." Amnesty International China researcher William Nee said China is merely demonstrating what Hong Kong has to fear from Beijing. "The rounding up of activists in mainland China only underlines why so many people in Hong Kong fear the growing control Beijing has in their city's affairs," Nee said in a statement. "The fundamental freedoms being exercised by hundreds of thousands of people in Hong Kong continue to be denied to those in mainland China." CHRD said some of those detained in China had made plans to join the protests in person. "Police in China have harassed and warned activists in many cities, concerned that they may try to travel to Hong Kong or take to streets to protest," the group said. "The occupation of several areas in Hong Kong, including parts of its financial and political center, has inspired many Chinese on the mainland and encouraged them to speak up for democracy," it said.