Renewed calls on the internet for protests have prompted Chinese authorities to mount a show of force.
Anonymous postings had urged people to stroll silently in areas of major cities, as a way of calling for change, the BBC reports.
The massive police deployments are being seen as a sign of the Communist Party's nervousness at the civil unrest and revolutions across the Arab world.
This was the third week of calls for protests and the anonymous posts urged people to take a walk through Xidan, a busy shopping area.
At Xidan and another shopping area, Wangfujing, there were hundreds of uniformed police; men posted every few yards. Reporters were banned from filming or interviewing anyone.
A BBC correspondent says data signals on mobile phones were blocked and huge numbers of plainclothes security men were everywhere.
The correspondent says crowds of shoppers were out but it was not clear if any were protesters.
In an area near Peking University, where online messages said a gathering of students may have been planned, police also closed down the subway and mobile phone networks, and police helicopters were reported hovering overhead.
In Shanghai, at least 17 foreign journalists were detained at the protest site, People's Square, for not having permission to be there.