Police in China’s capital said Tuesday they will start patrolling the Web using animated beat officers that pop up on a user’s browser and walk, bike or drive across the screen warning them to stay away from illegal Internet content.

Starting Sept. 1, the cartoon alerts will appear every half hour on 13 of China’s top portals, including Sohu and Sina, and by the end of the year will appear on all Web sites registered with Beijing servers, the Beijing Public Security Ministry said in a statement.

China stringently polices the Internet for material and content that the ruling Communist Party finds politically or morally threatening. Despite the controls, nudity, profanity, illegal gambling and pirated music, books and film have proliferated on Chinese Internet servers.

The animated police appeared designed to startle Web surfers and remind them that authorities closely monitor Web activity. However, the statement did not say whether there were plans to boost monitoring further.

The male and female cartoon officers, designed for the ministry by Sohu, will offer a text warning to surfers to abide by the law and tips on Internet security as they move across the screen in a virtual car, motorcycle or on foot, it said.

If Internet users need police help they can click on the cartoon images and will be redirected to the authority’s Web site, it said.

“We will continue to promote new images of the virtual police and update our Internet security tips in an effort to make the image of the virtual police more user friendly and more in tune with how web surfers use the Internet,” it said.

China has the world’s second-largest population of Internet users, with 137 million people online, and is on track to surpass the United States as the largest online population in two years.

The government routinely blocks surfers from accessing overseas sites and closes down domestic Web sites deemed obscene or subversive.