Police powers

The present discussion over the way the police dealt with the G20 protesters has raised in my mind the more general issue of the rights of the police when the commission of their duties impinges upon our lives. Often we hear news stories of police evacuating areas, or closing roads and such. What is the legal position here? Suppose I refused to leave my house when instructed by them to do so. Suppose I insisted on entering an area that had been sealed off - to get to my house or business perhaps. Other than possibly being done for obstruction (and I think that would have to be something more than simply disobeying them and would require some positive act on my part which prevented them from doing their job), have I committed any other offence? What right have they to proscribe my movements? Certainly as far as the G20 business is concerned, I would have thought this came perilously close to unlawful imprisonment - unless there is some legislation of which I am not aware.