A grassroots bloggers conference to be held in Shanghai has been cancelled after authorities decided it was too sensitive, participants said Saturday, as officials tighten control over social media.

It is the first time the Chinese Blogger Conference had been cancelled since it began six years ago, said Zhou Shuguang, who had booked the venue at the Shanghai Jiaotong University for the meeting this weekend.

Organisers were told by university officials on Friday afternoon that the Shanghai government had decided "the content of the meeting was sensitive" and could not be held at the proposed venue, Zhou, who is also a blogger, told AFP.

A "back-up venue" at the Shanghai University of Finance and Economics was also cancelled, Zhou said.

Rebecca MacKinnon, a US-based Internet analyst and blogger, confirmed to AFP that the meeting had been cancelled.

"The organisers were told they should cancel it so they did," said MacKinnon, who had travelled to Shanghai to attend the conference.

MacKinnon said participants would meet informally.

AFP calls to the universities were not answered and Shanghai government officials said they were not aware of the conference.

The cancellation comes after a Chinese woman was jailed to a year in a labour camp for retweeting a Twitter posting mocking anti-Japanese protesters and suggesting they attack the Japanese pavilion at Shanghai's World Expo.

Human rights groups said Cheng Jianping could be the first person in China persecuted over a tweet, after she was convicted last Monday of "disturbing social order".

It also comes as Beijing faces intense criticism over its tough response to the awarding of this year's Nobel Peace Prize to jailed dissident writer Liu Xiaobo, who is serving an 11-year sentence for subversion.

Liu's win last month prompted an angry response from Beijing, which has responded by muzzling his family members.

Social networking sites like Twitter and Facebook are blocked by the government, but many users access them on the mainland via virtual proxy networks.