BEIJING » China’s most popular microblogging site is cracking down on what it says is the spread of false rumors after the ruling Communist Party told Internet companies to tighten control over information online.

The move by Sina Corp. reflects the pressure on China’s Internet companies, most of which are privately owned, to take initiative to help Beijing enforce censorship or risk losing the right to operate profitable businesses in a fast-growing market.

Sina’s Weibo service has sent notices to its 200 million users denying two reports posted on the site, including one about the killing of a 19-year-old woman. It said the accounts of users who originated the reports were temporarily closed.

The move comes amid the ruling party’s most sweeping crackdown on dissent in years as it tries to prevent the rise of Middle East-style protests.

The party secretary for Beijing, Liu Qi, visited Sina’s headquarters last Monday and said Internet companies should block the spread of false and harmful information, according to a party newspaper. Liu gave no details of what the party wanted, but Sina issued a statement Thursday saying it would "put more effort into attacking all kinds of rumors."

Beijing encourages Internet use for education and business but worries about its potential to spread criticism of communist rule. Authorities tightened controls after social networking and other websites played a key role in protests that brought down governments in Egypt and Tunisia.

Weibo users reacted with dismay to Sina’s move.

"The constitution stipulates freedom of speech, but there is no freedom of speech in reality," said one posting on the site. "Please refute this rumor."

Sina’s announcement gave no indication it was following government orders but many user comments said it reacted to Liu’s visit.

Others questioned whether the ruling party would use the policy to suppress the spread of true information that might embarrass officials.

"Do not ban the truth through ‘refuting rumors,’" said one.

Web bulletin boards and Sina’s microblogging site are especially sensitive because they give China’s general public a rare opportunity to express opinions to a wide audience in a society where the party controls all media.

Communist leaders have allowed such services as they try to strike a balance between controlling information and developing an Internet industry they hope will help drive China’s modernization. Private sector Internet services are required to monitor content on their own and quickly remove any that violates censorship rules.

Sina CEO Charles Chao told Forbes magazine in March that Weibo had at least 100 employees monitoring content 24 hours a day.

In its announcements Friday, Sina denied a report on Weibo that a suspect in the slaying of a 19-year-old woman was freed in the central city of Wuhan because of his politically influential father. It cited police as saying the suspect was still in detention.

Sina said the account of the user who spread the report was suspended for one month.

The company also denied a report that the Chinese Red Cross was improperly charging hospital patients for blood. It said the Red Cross had no role in managing blood donations, only in assisting health authorities with publicity.

Sina said it has created a separate channel dubbed "Weibo Refutes Rumors" to spread denials of false information. It announced an email address for users to send reports of possible falsehoods.

China’s online population is by far the world’s biggest, with 485 million Internet users as of June 30, and is still growing rapidly, according to the government-sanctioned China National Internet Information Center.

That has created huge profit opportunities for Internet companies that comply with official controls, giving Beijing leverage to encourage cooperation and for private sector businesses to bear the cost of enforcing censorship.

Baidu Inc., which operates China’s dominant search engine, says its profit for the three months ending in June rose 95 percent over a year earlier to $252.6 million.

Baidu’s market share has grown since U.S.-based rival Google Inc. closed its China search engine last year after saying it no longer wanted to cooperate with censorship.

Sina, whose shares are traded on the U.S.-based Nasdaq stock market, reported a $10 million quarterly profit. That was down from $25.2 million for the same period of 2010 due to the costs of launching Weibo.

Another private company, Alibaba Group, has told merchants on its popular e-commerce platform Taobao to stop selling virtual private network and other software that can bypass Internet filters meant to block access to foreign websites deemed subversive or obscene.

Alibaba said last week it received no government orders and took the action after it found VPNs were being used to visit foreign websites illegally.

In a separate crackdown, the official Xinhua News Agency said Sunday that regulators have shut down 6,600 websites linked to companies that help clients delete unfavorable news items online or spread rumors to damage competitors.

Between April and June, authorities deleted more than 790,000 online posts and articles that contained illegal material, Xinhua said.

In one high-profile case, China’s biggest milk producer, Mengniu Dairy Group Co., said last October that one of its managers had tried to boost his sales by spreading rumors online that a rival’s infant formula might be dangerous.