BEIJING — China has begun censoring billions of telephone text messages to ensure that people do not use the popular communication tool to undermine one-party rule.

The campaign, announced on Friday by the official New China News Agency, comes after text messages sent among China's nearly 300 million mobile phone users helped to expose the national cover-up of the SARS epidemic last year. Text messages have also generated popular outrage about corruption and abuse cases that had received little attention in the state-controlled media.

It is a sign that while China has embraced Internet and mobile phone technology, the government has also substantially increased its surveillance of digital communications and adopted new methods of preventing people from getting unauthorized information about sensitive subjects.

This week, government officials began making daily inspections of short-message service providers, including Web sites and the leading mobile phone companies. They had already fined 10 providers and forced 20 others to shut down for not properly policing messages passing through their communication systems, the news agency said.

The dispatch said the purpose was to stop the spread of pornographic messages and false or deceptive advertising as well as to block illicit news and information.

All such companies are being required to install filtering equipment that can monitor and delete messages that contain key words, phrases or numbers that authorities consider suspicious before they reach customers. The companies must contact the relevant authorities, including the Communist Party's propaganda department, to make sure they stay in touch with the latest lists of banned topics, executives in the industry said.

Although text messaging is still in its infancy in the United States, it has become a primary means of communication in China. Chinese mobile phone users sent 220 billion text messages in 2003, or an average of 7,000 every second, more than the rest of the world combined, China Telecom data shows.

Many people with mobile phones like text messaging because it is quieter and less expensive than making phone calls. Messages can also be sent to multiple people at once and, at least until recently, were considered too unimportant or technologically difficult to monitor.

The authorities have become increasingly attuned to the threat posed by mobile messaging, as it has become not only a convenient way to talk and gossip, but also a competitor in the news business.

Phone messaging is faster and easier to access than chat sites on the Web, which have also become forums to disseminate information and opinions. China had already taken steps to monitor Web sites more carefully and had arrested several dozen "cyberdissidents" for posting articles or expressing views on the Internet that the authorities deemed unacceptable.