If ever you know, hear or see someone who needs assistance, then you should do something. You can help by making a phone call to police or an emergency service.
Physically or verbally intervening is a choice you need to carefully consider depending on the circumstances. If you are unsure, keep a safe distance until help arrives.
We all have a role to play in helping those that may be more vulnerable than ourselves. Everyone has a different perception of fear. Be aware and supportive of those who are having difficulties.
Never automatically presume help is not needed. If, for example, you see a screaming child being carried somewhere, don't always presume, as most people do, that the child is having a temper tantrum.
One option could be to ask the child "Hi there, is this your mum that is upset with you?" To dramatically change the situation the child might respond with "This is not my mother, she's taking me away!"