Sounds just like ds1. We feel unprepared because the 'terrible twos' didn't really happen and we thought we'd got away with it.
Since the summer he's been as you describe- just ridiculous screaming over the most minor of issues. He will be red-faced, in tears, hitting us (though deliberately very gently - he's careful not to actually hurt!) telling us how 'ridiculous' we are because we are going to the park and he hadn't finished playing (to give an example from today).
We do the usual things and give him fair warning of change of activities etc but it is very draining and there are times when I question what we are doing. What works for us (not works as in stops it, but doesn't make it worse) is humour, turing into a game, brisk jollying along. What invariably escalates the situation is getting angry, trying to enforce any kind of discipline.
As I said, I sometimes question what we are doing but my comfort is that his teacher (YR) says his behaviour is great, he is very polite and has excellent concentration. She is especially impressed since he didn't go to pre-school but has settled into all the routines etc really well. So it seems he saves all the horridness for us, which is certainly the way I would choose it to be.
So, no advice, just a massive hijack but just wanted to share and as I have been meaning to post this kind of thing for a while.
What triggers/improves yours?