Tantrums often seem to happen in shops. This can be really embarrassing, and embarrassment makes it extra hard to cope and stay calm. Keep shopping trips short. You could start by going out to buy one or two things only, and then build up from there. Once you've managed one quick trip without trouble, you're beginning to make progress.
Some parents find it helps to hold their child, quite firmly, until the tantrum passes.
This usually only works when your child is more upset than angry, and when you yourself are feeling calm and able to talk gently and reassuringly. Don't hit, bite or kick back. It makes behaving like that seem all right. You can still make it clear that it hurts. Children often go through patches of insecurity or upset and let their feelings out by being aggressive - at playgroup, for example.