How to clip/mask a filter?

Hello all,

I'm having a problem with implementing one of my ideas: I'd like to
have the whole page blurred, except a focus point - a circle in the
middle of the page. Additionally, the circle wouldn't have sharp edges
- it would have some kind of RadialGradient (lack of the filter inside
(sharp) and gradually blurring outside to fit the rest of the page)...

It wouldn't be a problem if that was not to be a blur filter, but a
color - simply masking. However...

Is it possible to "mask a filter region" or make a filter region to be
eg. circle (instead of the whole element)?

I would be really glad to hear any answers to my questions or just
hints where and what to look at to solve this issue.

Best regards
Marcin

PS. I hope it's an appropriate list...