A ban on Internet users commenting on posts to China’s two largest microblogging sites enters its third day after the government closed 16 websites and detained six people for spreading rumors of a coup attempt in Beijing.

Sina Corp. (SINA), the largest provider of the Twitter-like service with more than 300 million registered users, and Tencent Holdings Ltd. (700), the No. 2 player, announced the 72-hour suspension of the commenting function starting at 8 a.m. March 31, citing a need to clear up rumors and illegal information.

Speculation on the Internet on March 20 of a coup attempt helped spark the biggest jump in four months in the cost of insuring against a default on Chinese government bonds. That was five days after Bo Xilai, earlier seen as a contender for promotion to China’s top decision-making body, was removed as party secretary of the municipality of Chongqing.

“Although the punishment was not harsh, it does send a message to other Internet companies to be more cautious in terms of content control,” JPMorgan Chase & Co. analysts Dick Wei and Evan Zhou wrote in a note dated April 1.

Tencent fell 1.1 percent to HK$214.20 in Hong Kong trading as of 10:29 a.m. local time. Sina fell 1.1 percent to $65.00 in Nasdaq trading on Mar. 30.

Government oversight of microblogging services has tightened as they’ve grown in popularity. In December, China began requiring users in cities including Beijing, Guangzhou and Shenzhen to register their real names. That rule will be expanded to other regions, Wang Chen, minister of the State Council Information Office, said in January...