I started playing SR2 yesterday, and after escaping the town hall, the police kept chasing me, and I couldn't seem to lose them. I tried stoping and running them over when they got out of the car, and ramming their cars, but neither seemed to work, as more cops just showed up.
If it help, while in the town hall I shot a bunch of cops (which I think is required), then I grabbed a SWAT vehicle and tried to hightail it out of there.
Plowing through more cops is definitely not the way to reduce your Notoriety... You can either keep evading, and it will decrease over time, or you can visit the Forgive and Forget "drive through confessional". You can find them on the map.
Alternatively, you can also enter a story event. The story takes precedence over the open-world portion of the game.
In either case, it's best if you try to put a little distance between yourself and the cops instead of antagonizing them further. On more than one occasion, I had police get in front of me and cut me off, preventing me from actually entering the confessional or reaching an event site.
The linked wiki also references going to a plastic surgeon and changing your face, but I never tried that method so I don't know if it actually works.
