I spy

A story in today's New York Times describes a wide-ranging surveillance program begun before the Republican National Convention:

In the records reviewed by The Times, some of the police intelligence concerned people and groups bent on causing trouble, but the bulk of the reports covered the plans and views of people with no obvious intention of breaking the law.

The latter include church groups, theater companies, antiwar organizations, and anyone suspected of planning to protest at the RNC. Far from being a local operation, undercover police officers spied on people throughout the U.S., Canada, and Europe. They infiltrated groups, posed as fellow protestors at rallies, and videotaped participants at public events.
Where do we draw the line between promoting public safety and undermining lawful political activity?