Topics regarded as sensitive include names of political leaders
By Weiliang Nie
BBC Chinese Service
To police the internet China has employed what is regarded by many as the world's most formidable censorship machine. But as ever more Chinese get online, more and more users also understand what it means to jump over the government's "Great Firewall of China".
The country's firewall system is complex and multi-layered, says Isaac Mao, one of China's first bloggers.
"The first layer is technical. The government has invested a lot to build routers and other infrastructure to filter, monitor and block websites and look into how people communicate," he says.
Even an innocent children's song - I love Beijing's Tiananmen - can fall foul of the censors
"The second layer is the 'social layer'. It deploys many people to monitor and track people's activities online.
"The most important part is the third layer. I call it the 'psychological layer'. Because of the technical settings and internet police, internet users have become more self-censored, fearful of being tracked."
The impact of censorship is felt keenly by internet users in China, even affecting their everyday online search results, says Dean Peng, an independent commentator and columnist in Beijing.
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"Every day when I search for something which may be regarded as sensitive by the authorities, I encounter difficulties," he says.
"The search engine would tell me that the results that you are looking for will not be shown due to the local laws and regulations."
Topics regarded as sensitive include anything to do with Charter 08, an online campaign for democratic reforms launched by dissident professor Liu Xiaobo, who has since been jailed.
Other topics include the names of political leaders, banned religious group such as the Falun Gong, the restive province of Xinjiang and its Uighur people, Mr Peng says.
"And the blacklist changes from time to time," he adds.
The BBC conducted its own experiment, using China's most popular search engine, Baidu, and the results broadly confirmed the picture painted by Mr Peng.
Sometimes, the censorship can throw up real absurdities, according to Mr Mao.
Even an innocent children's song - I love Beijing's Tiananmen - can fall foul of the censors, he explains.
"Initially they block the word Tiananmen because it is associated with the Tiananmen crackdown 20 years ago," he adds. "Then they block the word Beijing because there are many things that happened in Beijing."
"The government also restarted the campaign against online pornography last year, so 'I love' also becomes a sensitive key word.
"Now, if you search for the song, the results returned can be: 'sensitive key word, sensitive key word, and sensitive key word'."
According to Willy Lam, former China editor of the Hong Kong-based South China Morning Post, China has every reason to control the internet.
"It is quite obvious looking from outside or the Western perspective that the Communist Party regime is quite stable. However the social and political situation in China is potentially unstable."
There are an estimated 100,000 cases of mass incidents - riots and disturbances - every year, Mr Lam says.
And the gap between the rich and poor is widening, resulting in "a severe situation of class antagonism between the newly rich entrepreneurs, senior party leaders on the one hand, and the so-called disadvantaged sectors - the peasants, migrant workers", he adds.
"The erection of the so-called 'Great Firewall of China' is a pre-emptive strike against possible potential destabilising factors getting worse, particularly given Beijing's suspicion that there are so-called anti-China organisations in the West, in the US in particular, who want to exploit these potential destabilising factors in China to make trouble for the regime."
But the firewall is not unbreakable, at least to some Chinese internet users.
"Just two years ago, only 5% of Chinese internet users knew that the government censored the internet," Mr Mao says.
"But today, information flows faster and faster and people try to use different tools to spread information between social networks."
"There are a minority of users who can use technology to bypass censorship. No more than one or two percent. More users - about 18% - have become second-hand information consumers from those savvy users."
"So roughly 20% of Chinese internet users now understand what 'Fan Qiang' ('circumventing the firewall') means, and they also have a strong determination to do so."
Stopping people evading the firewall is not easy, says Zhou Shuguang, an active Twitter user and blogger from the central province of Hunan.
Tor helps users defend against network traffic analysis by security services
He cites the availability of free, open-source, peer-to-peer (P2P) software such as Tor.
"Everybody can use it. If you can pay some money, you can get a virtual private network (VPN) account so you can get a faster connection to the internet."
According to internet users in China, you can download the P2P software from the Tor website. However, the site itself has been blocked in China.
Nevertheless, more and more so-called "mirror sites" - exact copies - are now emerging.
"They can block one. Maybe five will appear tomorrow," Mr Mao says.
You can listen to Weiliang Nie's radio documentary for the BBC World Service