A severed finger sparked an online uproar that went viral. And very quickly, rattled authorities here took note.

The story of Sun Zhongjie, a 19-year-old driver who chopped off his finger to decry police entrapment, shows how the Internet has become an effective tool of public protest in this tightly controlled country.

Almost every form of open dissent is outlawed in China, but mass protests organized online are increasingly putting pressure on police, judges and other officials -- and getting results.