Although there were no reports of large demonstrations, the outsize government response highlighted Beijing’s nervousness at a time of spreading unrest in the Middle East aimed at overthrowing authoritarian regimes.

 The words “jasmine revolution,” borrowed from the successful Tunisian revolt, were blocked on Twitter-like blogging sites and Internet search engines while cellphone users were unable to send out text messages to multiple recipients. A heavy police presence was reported in several Chinese cities.

 In recent days, more than a dozen lawyers and rights activists have been rounded up, and scores of dissidents have reportedly been placed under varying forms of house arrest. At least two lawyers are still missing, family members and human rights advocates said Sunday.

 In Beijing, a huge crowd formed outside a McDonald’s in the heart of the capital on Sunday after messages went out listing it as one of 13 protest sites across the country. It is not clear who organized the campaign, but it first appeared Thursday on Boxun, a Chinese-language Web site based in the United States, and then spread through Twitter and other micro-blog services.

 By 2 p.m., the planned start of the protests, hundreds of police officers had swarmed the area, a major shopping district popular with tourists.

 At one point, the police surrounded a young man who had placed a jasmine flower on a planter outside the McDonald’s, but he was released after the clamor drew journalists and photographers.

 In Shanghai, three people were detained during a skirmish in front of Starbucks, The Associated Press reported. One post on Twitter described a heavily armed police presence on the subways of Shenzhen and another claimed that officials at Peking University in Beijing had urged students to avoid any protests, but those reports were impossible to verify Sunday evening.

 The messages calling people to action urged protesters to shout “We want food, we want work, we want housing, we want fairness,” an ostensible effort to tap into popular discontent over inflation and soaring real estate prices.

 In a sign of the ruling Communist Party’s growing anxiety, President Hu Jintao summoned top leaders to a special “study session” on Saturday and urged them to address festering social problems before they become threats to stability. “The overall requirements for enhancing and innovating social management are to stimulate vitality in the society and increase harmonious elements to the greatest extent, while reducing inharmonious factors to the minimum,” he told the gathering, according to Xinhua, the official news agency.

 Human rights advocates said they were especially concerned by the recent crackdown on rights defenders, which intensified Saturday after at least 15 well known lawyers and activists were detained or placed under house arrest. Several of them reached by phone, including Pu Zhiqiang and Xu Zhiyong, said they were in the company of security agents and unable to talk, while many others were unreachable on Sunday evening. Two of the men, Tang Jitian and Jiang Tianyong, remain missing.

 Many of those subjected to house arrest had met in Beijing on Wednesday to discuss the case of Chen Guangcheng, a blind lawyer under strict house arrest in rural Shandong Province. The plight of Mr. Chen and his family gained widespread attention last week after a video he and his wife made about his arrest emerged on the Internet.

 One of the missing lawyers, Jiang Tianyong, was forced into an unmarked van on Saturday night, his second abduction in recent days, his wife, Jin Bianling, said by telephone. She said the police had also searched the couple’s home and confiscated his computer and briefcase.

 After his first detention on Wednesday, Mr. Jiang said that he was taken to the police station and assaulted.

 Most of those who thronged the McDonald’s in Wangfujing, the Beijing shopping district, said they had no idea what the hubbub was about. Some thought that perhaps a celebrity had slipped into the restaurant for a hamburger. But a young man, a Web page designer in his late 20s, quietly acknowledged that he was drawn by word of the protest.

 Despite the absence of any real action, the man, who gave only his last name, Cui, said he was not disappointed by the outcome, in which police officers tried in vain to determine who was a potential troublemaker and who was simply a gawker. He predicted that many people, emboldened by the fact that an impromptu gathering had coalesced at all, would use social networking technology to stage similar events in the future.

 “It’s very difficult to do this in China, but this is a good start,” he said. “I’m thankful to be able to participate in this moment in history.”

 Zhang Jing, Jon Kaiman and Jonathan Ansfield contributed research.

 http://www.nytimes.com/2011/02/21/world/asia/21china.html?_r=1&ref=china