Always remember demonstrators want to be seen, otherwise they wouldn't be demonstrating, so the more coverage they can get the better, for them that is.
No doubt there will be 'two sides' to the event so be careful that you don't give the impression that you are on one side or the other, try and be seen to be neutral. Another tip is to show a great deal of interest in the placards that they carry and from that you will be able to move on to the demonstrators' faces, if that what interests you.
I've always found that the biggest problem is the police. They have a job to do, normally with the kind of demo you are talking about that means keeping the two sides apart if things turn ugly, so if they tell you to do something my advice is 'DO IT'.
Most demos are carried out in a carnival atmosphere but some can be very intimidating, the G20 last year in particular.
The most menacing I've attended was a meeting of The Black Muslims, followers of Louis Farraknd, in Hyde Park
a couple years ago. It might at first sight looked extremely menacing but it turned out they were just a bunch of posers in dark suits trilby hats, dark glasses and bow ties. Mind you I didn't tell them that!
