SHANGHAI—Organizers were forced to cancel an annual blogging conference in Shanghai this weekend under pressure from authorities, the latest sign of tightening limits in China on free expression.

The Chinese Blogger Conference has attracted dozens of prominent online commentators, entrepreneurs, digital artists and others each year since it was started in Shanghai in 2005. Many of the attendees are critical of government censorship, so the event is considered potentially sensitive.

This year, organizers waited until four days ahead of the two-day conference's planned start on Saturday to announce the venue, an office building in Shanghai's Xuhui District, near Shanghai Jiaotong University. But the planned hosts reneged late last week owing to pressure from authorities not to let their venue be used for the conference, according to one of the organizers.

It couldn't be determined which arm of the government was responsible. A person who answered the phone Sunday at Shanghai's cultural affairs bureau, which oversees events being held in the city, declined to comment.

China's government has steadily stepped up efforts in recent years to curb free expression on the Internet, which has more than 420 million users in China—the most of any nation.