A Beijing city regulation clamping down on people who send text messages that "spread rumors" or "endanger public security" is a threat to freedom of expression, a watchdog group said Monday.

China Human Rights Defenders, an international network of activists and rights monitoring groups, said the recent regulation on text messages "raises serious concerns over the restriction of freedom of expression in China."

The group said in a statement that an average of 180 million text messages are sent every day and that text messaging has become one of the most important means of receiving information unavailable in the mainstream media.

The 2008 Olympics, which Beijing is hosting, offer a high-profile opportunity for protesters to air their grievances against China on issues like religious freedom, human rights and Tibetan independence.

Beijing police will work with government agencies and telecommunications companies to investigate and punish those using text messages to "spread rumors" or "endanger public security," the city government said in a notice posted on its Web site late last month.

Chinese authorities commonly use vague charges such as those to detain dissidents or others it views as a threat to the ruling Communist Party.

Although the notice did not detail specific punishments, a story in the city's Communist Party mouthpiece newspaper, the Beijing Daily, earlier this year said people who spread rumors or other false information are subject to detention for up to 10 days and a fine of up to $70.

China has more than 500 million cell phone users and text messaging has become an increasingly effective way to spread word of meetings or demonstrations.

This summer, plans to build a chemical plant in the southern coastal city of Xiamen were suspended after residents sent nearly 1 million text messages to friends and family, urging the government to abandon the project because of its alleged health and environmental risks.

Meanwhile, a Tibetan language online discussion forum was shut down this month for having content that was against Chinese law, according to a notice on its Web site.

The popular forum, which was hosted at www.Tibet123.com, was shut down for containing "illegal content," according to a notice on the Web site.

The notice says it "strongly condemns the 'rotten apple in the barrel' who published harmful information."

The notice then invites people to leave comments. It is not clear if the notice is from the site moderator or the government.

Media rights group Reporters Without Borders has called the site "the most dynamic forum in the Tibetan blogosphere" with over 6,200 registered members.

The Paris-based group said the site has been closed since Dec. 6. It was still inaccessible Monday.