The moves underline official anxieties ahead of this year's leadership transition, particularly since the sacking of Chongqing party secretary Bo Xilai led to widespread speculation about infighting at the top.
As the mood on microblogs grew increasingly febrile, there were even claims of an attempted coup in the Chinese capital – complete with photographs of military vehicles that turned out to be from a parade three years ago.
State news agency Xinhua said Beijing police detained six people for spreading rumours of "military vehicles entering Beijing and something wrong going on in Beijing". Citing a spokesman for the state internet information office, it said the claims were "fabricated by some lawless people" and had been a bad influence on the public.
The office also closed 16 websites for allegedly spreading the rumours.
The spokesman said that Sina and Tencent, the organisations which operate China's most popular versions of Twitter, and which each have hundreds of millions of microbloggers, had pledged to strengthen their managements after being "criticised and punished". The two firms have disabled their comment functions for three days.
Internet users have become increasingly bold in their willingness to discuss current affairs and even sensitive political news, prompting officials to seek new ways of reining them in. From two weeks ago real name registration is supposed to be in force for all users of the Sina and Tencent services, although several users say they have posted material without having given their details.
"By falsely packaging lies and speculation as 'truth' and 'existence', online rumours undermine the morale of the public, and, if out of control, they will seriously disturb the public order and affect social stability," said a commentary in the People's Daily, the official Communist party newspaper.
"Tackling 'rumours' is the way they are sugar-coating this intensified control. It seems relentless," said David Bandurski, of Hong Kong University's China Media Project. "This is a phenomenon we will probably see through the rest of the year. For China this is the ultimate sensitive year.
"I have no doubt they will keep exploring ways to disrupt the conversation on [microblogs] as much as they have to [by] keeping control of discussion on sensitive issues and speculation, some of it well-grounded."
With original posts still allowed, tens of thousands of microbloggers left comments attacking the decision.
Investor Wang Ran told his 1.5 million followers over the weekend: "What happened today reminds us again how important, imperative and remote is the use of law to rule China.
"Without using law to rule China, we can't avoid danger, or absurdity."
Property tycoon Zhang Xin, who has more than 3 million followers, wrote: "What is the best way to stop 'rumours'? It is transparency and openness. The more speech is discouraged, the more rumours there will be."
"The underlying problem is that you can't get the truth out of the government, so you might as well believe stuff flying around on the internet," agreed Jeremy Goldkorn, who runs the Danwei website on Chinese media. "But what this does is remind everyone who is in charge … Sina and Tencent are going to be pretty cautious and I think will be stepping up censorship: this is a shot across the bows."
Some have asked why rumours about senior political leaders – particularly Bo – have circulated for so long recently, given that censors are usually quick to delete such speculation.
"I do get the sense that some things have been tolerated that perhaps in other times would not be. Certainly, it seems it's been allowed that enough has been circulated about Bo Xilai to blacken his name," said Goldkorn.
In a separate move, police in Beijing said they had arrested more than 1,000 suspects, deleted more than 208,000 "harmful" online messages and punished 70 internet companies that defied warnings in a crackdown on web-related crimes. The spokesman said that the "Spring Breeze" campaign focused on information relating to smuggling firearms, drugs and toxic chemicals, as well as the sale of human organs, counterfeit certificates and invoices, and trade in personal data.
