The Democratic and Republican conventions in Boston and New York City pose challenges on a scale police have not seen before.

Police must balance demonstrators' rights of free speech against the conventions' rights of free assembly. The law says protesters must be close enough to the action to be heard but not close enough to harass.

At the conventions four years ago, most demonstrators just demonstrated, but some threw bottles, assaulted cops, overturned patrol cars and stole property. But out of 600 arrests, less than 5 percent were prosecuted, mostly because prosecutors didn't want protesters to clog the courts and use their cases as a platform for their cause.

"Processing hundreds of people for misdemeanors is not cost-effective," said Bristol, Mass., District Attorney Paul Walsh. "The system has to triage. We have rape, robbery and murder cases."

But prosecutors say that's changing. Anarchists (search) who have vowed to disrupt the conventions this year are now promising to cripple police horses and soak their clothing in explosive residue to trick bomb-sniffing dogs.

These acts will turn the anti-corporate crusaders from a mere annoyance to a threat that could divert police from preventing real attacks. Police aren't taking any chances this year, and may be less tolerant of lawbreakers.

"It's clearly an opportunity, if you were a terrorist so inclined," said Miami Police Chief John Timoney. "This creates, not so much a perfect storm, but it certainly creates a perfect opportunity."

Click on the video box at the top of this story to watch a report by FOX News' William LaJeunesse.