China is launching its strongest official measure yet to quell electronic expressions of discontent, clamping down on its versions of Twitter that have increasingly fueled once-rare protests and threaten to undermine its leaders' firm hold on power.

In the name of defending Chinese cyberspace against "harmful information," the Beijing city government announced new rules likely to chill a raucous national conversation on services like Sina Weibo, to which Chinese users are flooding to share brief text messages, photos and...