Chinese police have arrested six people and shut 16 websites after rumours were spread that military vehicles were on the streets of Beijing, officials say.
The web posts were picked up last week by media outlets around the world, amid uncertainty caused by the ouster of top political leader Bo Xilai.
The country will begin a once-in-a-decade leadership change later this year. But one of the main contenders for promotion - Bo Xilai - has just been sacked, suggesting a fierce fight behind the scenes for control of the ruling Communist Party.
Mr Bo was removed from his post amid allegations that his police chief and former ally had tried to seek asylum at a US consulate.
Chinese censors had previously blocked searches on various sites for terms linked to Mr Bo.
There have also been lurid, and unsubstantiated, rumours that Mr Bo's fall was also linked to the death of a British businessman, Neil Heywood, who last year was found dead in a hotel room in Chongqing, the city where Mr Bo was Communist Party chief.
The State Internet Information Office (SIIO) said the rumours had a "very bad influence on the public".
Two popular microblogs have temporarily stopped users from posting comments.
The two sites, Sina Weibo and Tencent Weibo, are still letting people post to their own sites. But they said commenting on other people's posts would be disabled between 31 March and 3 April, so they "could act to stop the spread of rumours".
A spokesman for the SIIO told state news agency Xinhua earlier that the two websites had been "criticised and punished accordingly".
To be completely correct we should say we do not know what's going on. The fact is there is no evidence of a coup. But it is a subject that has obsessed many in China this week.
Welcome to the weird and wonderful world of reporting on China in the past few days. Coup rumours ricocheted back and forth, most over the internet, but some were picked up by western newspapers. China's microblogs were awash with speculation. Hard facts were non-existent.
Purges and power-struggles
Photographs of tanks and armoured cars on city streets were flying around Twitter and elsewhere. On closer inspection though, some of the pictures seemed to be old ones from rehearsals for military parades, others did not even seem to be of Beijing, as they claimed, but different Chinese cities.
There has been no evidence to substantiate the coup allegations but the authorities considered them damaging enough to take this extreme action.
Internet forums, microblogging sites, are perhaps the only area in which people could freely express their views, and many have done so anonymously.
In a country where there is very little information from the authorities, rumours take on an added value in a way they perhaps would not in other countries.