The message, posted Saturday on a foreign site called Boxun, was titled "The jasmine revolution in China," a reference to the revolution in Tunisia, The Guardian reported. It urged demonstrations in 13 cities.
It was unknown who posted the message, and some activists seemed unaware of it when they were arrested.
Police turned out in force at the designated locations, but a few people did show up. In Beijing, officers tried to arrest a man holding jasmine flowers but let him go in the presence of journalists. Xinhua said police detained three people after crowds gathered at People's Square in Shanghai.
The Information Center for Human Rights and Democracy in Hong Kong estimated that more than 100 activists across China were arrested, kept from leaving home or were missing.
At a Beijing meeting, President Hu Jintao told officials to "improve social management capabilities" and control of the Internet "to guide public opinion," Xinhua reported.
Wang Songlian of the Chinese Human Rights Defenders said, "It really shows us how much the government has identified with regimes in the Middle East."