Yeh, any protest is likely to be hi-jacked by extremists. If there was a guarantee of peaceful protest I would attend myself, because I think it's important that Mr Trump realises not everyone around the world thinks he's a matinee idol. And as for it not being our business what a democratically elected leader does or says - nonsense. It's the exact opposite. In a democracy (like the UK) we can express our feelings, within the law, about anybody or anything.
As its stands, Mr Trump is likely to be able to dismiss extreme protest for what it is. Sensible voices opposed to what he stands for will be lost. After his cosy interview with Mr Morgan he will continue to believe he's a superhero, opposed only by a 'mindless few'.