China's move to censor its two main microblogs and arrest people over rumours of a coup reveal a growing nervousness ahead of its first leadership transition in the social media age, analysts said Monday.

Late last week, authorities shut down 16 websites, arrested six people and announced new curbs on the country's two most popular microblogs after rumours of tanks and gunfire on the streets of Beijing swept the Internet.

The speculation followed the shock move to sack Bo Xilai, a rising political star whose removal analysts say exposed rifts in China's ruling Communist Party at a crucial time, ahead of a 10-yearly transfer of power.

"The authorities are really worried about this, they are definitely ratcheting up the pressure on the Internet," David Bandurski, who runs the China Media Project at the University of Hong Kong, told AFP.

"The authorities are telling people, 'watch what you say'. They are saying, 'we are serious about controlling the media and guiding public opinion'."

Bo was the Communist Party chief of the southwestern metropolis of Chongqing until he was sacked last month, ending his hopes of joining the Politburo Standing Committee, the elite group of leaders who effectively run China.

All but two of the nine places on that committee are expected to change hands later this year when President Hu Jintao and other top leaders step down from their Communist Party posts in a generational handover of power.

The transition will be the first to take place under the glare of China's weibos, microblogs similar to Twitter that have have taken the country by storm in recent years.

China, which has the world's largest online population with over half a billion users, blocks content it deems politically sensitive as part of a vast censorship system known as the Great Firewall.

But the number of weibo users has more than trebled since the end of 2010, according to government data, and the speed with which the microblogs have taken off has made it impossible for government censors to keep up.

Authorities had already been tightening controls on the microblogs, but the new curbs are the toughest yet, with the two leading weibos banning people from commenting on other users' posts for three days.

Li Datong, the outspoken former editor of Communist Party newspaper the China Youth Daily, said the crackdown was aimed at maintaining social stability until the leadership change, but cast doubt on its effectiveness.

"They will maintain this abnormal state of stability until the 18th Party Congress, and any voice that is different will not be allowed," said Li, who now works as a researcher for the publisher of the same newspaper.

"This so-called cleaning up of the Internet shows that China has more and more become a police state that must completely depend on police violence to control public opinion."

More than 1,000 people have been arrested since February for unspecified Internet crimes in Beijing alone, while over 3,000 websites have been warned over suspected wrongdoing, the official Xinhua news agency said last week.

Meanwhile, China's state-run papers have published a series of editorials warning against the damaging effects of online rumours and said those responsible for "lies and speculation" should be punished.

"The authorities are revealing their hopes to control public opinion, but differing opinions are always going to exist," said Ai Xiaoming, a professor at Zhongshan University in southern China and well-known government critic.

"This may be effective in the short term, but more and more people are able to get over the Great Firewall, where they can access all kinds of information."