China's two most popular microblogs, Sina Weibo and Tencent QQ, on Saturday blocked web users from posting comments on the sites, saying they were acting to stop the spread of rumours.
The Internet giants said the measure would remain in force until Tuesday. It came just hours after authorities said they had shut down websites they accused of spreading misinformation and made a string of arrests.
China has been strengthening its control of the Internet after being rattled by a flood of online rumours and criticisms.
Groundless rumours of a coup in Beijing began circulating on the Internet earlier in March following the dismissal of political star Bo Xilai.
"Rumours and illegal, harmful information spread via microblogs have had a negative social impact and the comments contain a large amount of harmful information," said a message on Tencent's website.
"From March 31, 8:00 am to April 3, 8:00 am, Weibo's comment function will be temporarily suspended," said Sina, which runs China's most popular microblogging service, Sina Weibo.
Authorities early Saturday said they had shut down 16 websites for spreading rumours of "military vehicles entering Beijing and something wrong going on in Beijing," the official Xinhua news agency quoted a spokesman for the State Internet Information Office as saying.
Six people had also been detained for "fabricating or disseminating online rumours," the State Internet Information Office and Beijing police said, according to Xinhua.