The New York Times offers "Police Infiltrate Protests, Videotapes Show". It's certainly worrisome if true.

Unfortunately, the "if true" part is key, since they have no actual proof that those discussed in the article were actually police or working with them. The allegations are based on a series of videotapes taken mostly by the protesters themselves, and all the proof they have is that the supposed infiltrators weren't cuffed, or were immediately released, or had a walkie-talkie. Nevertheless:

...The officers hoist protest signs. They hold flowers with mourners. They ride in bicycle events. At the vigil for the cyclist, an officer in biking gear wore a button that said, "I am a shameless agitator." She also carried a camera and videotaped the roughly 15 people present.

Beyond collecting information, some of the undercover officers or their associates are seen on the tape having influence on events. At a demonstration last year during the Republican National Convention, the sham arrest of a man secretly working with the police led to a bruising confrontation between officers in riot gear and bystanders...