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.

New regulations on messaging appear to have been phased in during recent weeks. Some mobile phone users said they had had trouble sending ordinary text messages around the 15th anniversary of the June 4, 1989, crackdown on democracy demonstrations in Beijing, perhaps because of tighter policing of the service.

One user said that messages he sent that included the numbers 6 and 4 in close proximity were never delivered, perhaps because they were screened as a possible reference to the date of the crackdown.

Wang Hongwei, a 25-year-old air-conditioning technician in Beijing, said he got up to 100 text messages every day — from friends, colleagues and news sites. He said he had found the service slower and less reliable recently, although he had not heard of the new monitoring orders.

"I don't think there's any justification for filtering every single message," he said. "The government should not be deciding what people say to each other."

Industry experts say message filtering technology is relatively straightforward, much like programs to block junk e-mail. The challenge is to provide robust software that can process enormous volumes of text messages without reducing their efficiency.

"You can filter as much as you like, just like a list of words," said Wang Yuanyuan, a sales manager at Venus Info Tech, which sells filtering software to Chinese messaging service providers.

She said the new rules would lead to heavy demand for her company's product.

"I think with the new rules the government will be expecting service providers to govern their content in a more regularized way, and this is what our system can do," she said.