After weeks of Internet-fueled rumors suggesting fissures in the top leadership ranks, Chinese authorities struck back this weekend, closing 16 Web sites and arresting at least six people in a broad crackdown on the freewheeling world of cyberspace.

Xinhua, the official Chinese news agency, said in a dispatch late Friday that the Web sites were closed, and the unnamed individuals detained, for “fabricating or disseminating online rumors.” For the past two weeks, the Internet has been filled with rumors of an internal power struggle after the largely unexplained March 15 ouster of the popular provincial Communist Party chief Bo Xilai.

Xinhua also said Saturday that the two most popular Twitter-like microblogging sites, “weibo.com” run by Sina and “t. qq” run by Tencent, had suspended their comment functions, “after they were punished for allowing rumors to spread.” The suspension of the user comments function was said to last until next Tuesday.

The State Council Information Office, which announced the new moves, said it took action against the sites for spreading rumors of “military vehicles entering Beijing and something wrong going on in Beijing.”

The unsubstantiated rumors included reports of a coup in Beijing, and gunshots being fired near the Zhongnanhai leadership compound. They were quickly dismissed as fabricated but still spread rapidly in the absence of much official information. China’s censorship authorities immediately began blocking certain search terms, such as “coup” and “gunshots.” But this weekend’s actions suggested that a broader effort was now underway to rein in the Internet.

Also on Saturday, the government announced that since mid-February, 1,065 people had been arrested, and 3,177 Web sites shut down, in a sweeping Internet crackdown called “Spring Breeze” that it said was aimed at stopping Internet-based crime such as counterfeiting, smuggling and identify theft.

The government said about 208,000 “harmful” online messages had been deleted, and 70 Internet companies had received “administrative punishment,” including some — it did not say how many — that had been shut down.

Xinhua referred to the crackdown as an effort to “cleanse” cyberspace.

In the past two years, the microblogging sites have emerged as an explosively-popular new free speech platform in China, surpassing the heavily censored traditional media as the main source of news — and indeed rumors — for tens of millions of Internet users, called “Netizens.”

The Communist authorities have alternately tried to co-opt the new technology — with government departments and party chiefs setting up their own microblogging accounts — while also seeking to suppress the relatively open discussion of topics once considered taboo.

Some microbloggers have been arrested, and the comments sections on sensitive postings are regularly disabled. The government recently announced plans to have all microbloggers register with their real names and identity card numbers, to make it easier to track down, and punish, people who post items that run afoul of the regime’s censors.

But generally the Netizens have managed to stay a step ahead of the censors, often by finding creative ways to talk about sensitive topics, such as using nicknames for powerful personalities, or words that rhyme with banned search terms. The censors at times seem to struggle to adopt to the quickly-changing technology and vocabulary, but one industry insider said he doubted that there would be an outright ban, because the social networking sites had become too popular.

At the beginning of the year, the China Internet Network Information Center estimated China’s Internet population to be a little more than 500 million users, with a penetration rate of nearly 40 percent of the population. At the end of November, it said, there were about 300 million microblog users.