BEIJING » China is closing a dozen websites, penalizing two popular social media sites and detaining six people for circulating rumors of a coup that rattled Beijing in the midst of its worst high-level political crisis in years.

The extensive clampdown, announced late Friday by state media, underscores the authoritarian government’s anxieties over a public that is wired to the Internet and eager to discuss political events despite censorship and threats of punishment.

A Xinhua News Agency report said Beijing police questioned and admonished an unspecified number of Internet users and detained six people not further identified. Aside from the 16 websites shut down, two Twitter-like services run by Sina Corporation and Tencent Holdings, which each have more than 300 million users, said they would disable their comment functions for three days in a necessary "concerted cleanup."

Users of the services — the most unfettered media in China — received a notice when they tried to post comments explaining the suspension was due to the many "rumors and such illegal, harmful information." Original postings are still allowed, and some used them to castigate the government.

"Every time you murder free speech, we all take note of your reaction. Your gradual loss of public credibility is the result of such messes," Wang Gongquan, a private equity executive, wrote on his Sina microblog account, which has 1.3 million followers.

Both the coup rumors and the crackdown show how the firing two weeks ago of Bo Xilai, the populist leader of the mega-city of Chongqing, has brought leadership struggles out of the usually closed confines of elite Communist Party politics and into the public.

Yet to be fully explained, Bo’s dismissal came after a top aide fled temporarily to a U.S. consulate, apparently to seek asylum and in violation of party rules. It also came as the senior leadership gears up for a handover of power to a younger generation leaders in the fall, always an a period of intense bargaining.

Politically minded Chinese saw the removal of Bo, considered a contender for a top job only months ago, as a sign of divisive infighting. Speculation about Bo’s fate and that of others ricocheted across microblogs and spiraled into talk of troop movements and gunshots around the leadership’s Zhongnanhai compound in central Beijing on March 19.

The Internet sites and people punished fabricated and disseminated rumors that "military vehicles are entering Beijing, something is going wrong in Beijing" and similar posts, Xinhua said, citing the State Internet Information Office, an interagency body charged with policing the Internet.

Over the past week, President Hu Jintao and other senior leaders have tried to project an image of unity and reassert control over the public message. Censors have furiously blocked and unblocked a range of terms to quell errant microblog chatter; "coup" remained a banned search term Saturday.

"Internet rumors and lies packaged as ‘facts’ will turn conjecture into ‘reality,’ stir up trouble online and disturb people’s minds," the party’s flagship newspaper People’s Daily said in a commentary accompanying the announcement of the clampdown. "If allowed to run amok, they will seriously disrupt social order, affect social stability and harm social integrity."

The Twitter-like microblogs, with their huge user numbers and rapid-fire postings, have proved a challenge for Beijing and its Internet monitors. Social media played a role in forcing the government to disclose more information after a crash on the showcase high-speed rail network last summer and to provide more information on the thick air pollution in Beijing and other cities.

A new rule requiring social media users to register accounts under their real names and identity card numbers by the middle of March failed to deter the rumors over Bo or a coup.

Following the detentions, shutdowns and other penalties, Pan Shiyi, a celebrity real estate developer and prolific microblog poster with 9 million followers, questioned Saturday whether the tactics were "the right medicine" to deal with rumors. Pan had not directly posted about a coup but had noted the heavy-handed censorship. To dispel rumors about himself, he said: "I’m not among the six."