The code of conduct, first announced earlier this month, stipulates that users of Sina's Weibo microblogging site cannot post information that is against the principles of the constitution, cannot harm national unity, disclose state secrets or publish false information, among other rules.

Many users said the restrictions were aimed at muzzling the often scathing and anonymous online chatter in a country where the Internet offers a rare opportunity for open discussion.

The move, the latest in a series of steps to rein in discussion on Weibo, comes as China prepares for a once-in-a-decade leadership handover, expected to be announced at a party congress later this year.

Sina, the biggest of the Weibo operators, also introduced a points system in which a user starts with 80 points and loses points for every violation. A score of zero results in a cancelled account. A user can gain points for validating his or her real identity.

"It gives Sina a firmer basis for expanding a ban on whatever is considered sensitive news," said one prominent Weibo user who spoke on condition of anonymity.

"The definition of what counts as sensitive was always loose and it's expanding all the time. Of course, nowadays, they're worried because of all the scandal and rumors before the 18th Congress."

In a sign of how intensely sensitive the issue is for the ruling Communist Party, censors blocked online searches for the name of Bo Xilai, the former Chongqing party boss cast out of the party's Central Committee.

Internet users have skirted restrictions on Weibo by using code words to discuss the issue. Sina employs technicians to scrub Weibo of politically sensitive posts.

Sina's move to implement a user contract comes after Beijing demanded last December that microblogging operators ensure their users are registered with their real names.

The company, which has invested heavily in Weibo, intends to start making money from it this quarter. Sina has said validating the real identity of Weibo users will be hard, given its roughly 300 million users.

Some Weibo users expressed dismay over the new restrictions.

"New rules will be carried out on the 28th," one wrote. "So I should publish my personal opinion on matters of state sovereignty, territorial integrity and social problems before then. After the 28th, I will just write about personal things."

(Reporting Melanie Lee, Sabrina Mao and Chris Buckley; Editing by Nick Macfie)

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