Chinese authorities began to implement new controls on the content of text messages sent to mobile phones this week, according to service providers.
Documents showing lists of hundreds of banned keywords, including "democracy", "human rights," and "telling the truth," were circulating widely on the Internet.
"Following the recent ruling by [telecoms service provider] China Unicom on forbidden keywords in text messages, we have undergone a cleanup of banned words in text messages," said a statement posted online by a number of Internet and telecoms service providers on Wednesday.
"Please would customers who use our text messaging platform check carefully, so as to avoid any situation in which they are unable to send messages," it said.
An employee who answered the phone on Wednesday at the Guangdong branch of Internet service provider now.cn confirmed that the lists of keywords were available on its website for public download.
"You can access our company home page from the text messaging platform, where you will find the illegal keywords for download," he said.
"This is a requirement being imposed by the telecoms [service provider], that these keywords cannot be sent. If they are sent, then they will not be received by the user," the employee said.
"The telecoms [service provider] ... told us they were acting on orders from leaders higher up," he added.
"[The requirements] have existed for a while now, but there weren't so many illegal keywords before."
A second employee at the same company confirmed that the use of banned keywords could lead to the termination of service.
"If you have been clearly informed, and yet you still keep using them, then it could lead to your phone being cut off," the employee said.
The documents were circulated widely among Chinese users of microblogging sites like Twitter.
Chinese netizen Zhi Zunyu commented: "We can see from this list of banned words what the government is afraid of. Whatever they fear, they ban."
User Zizhen told RFA: "This will affect normal communications."
The banned terms are divided into categories, including more than 700 relating to national security and politics, including “Cultural revolution,” “Xinjiang independence,” "June 4," and “anti-corruption.”
Other banned terms include "Chinese Communist Party," "petitioner," and "RFA."
Other keywords include references to the banned Falun Gong spiritual movement and to sexual activity, including the words "sex," "prostitute," and "lonely."
Around 800 terms referencing historical figures, political leaders, and dissidents were also banned, along with commercial spam, sensitive English words, and some number combinations, like the sensitive anniversary of June 4, 1989, expressed as 8964.
Reported by Xin Yu for RFA's Mandarin service. Translated and written in English by Luisetta Mudie.