I was on the Tube in London and all the seats were taken and a man was strap hanging a bit further down the carriage. The tube stopped and a heavily pregnant woman got on. I stood up and motioned to her to come over and take my seat. From me standing up and moving, the man raced down the carriage and plonked himself in my seat, with me still standing there. I opened my mouth to say something, when another woman sitting said 'oi mate, she stood up to give it to that lady there.' and we all stared at him until he got up.
Similar thing happened whilst on a busy bus. Saw a heavily pregnant woman get on so got up and made eye contact with her to let her know she had my seat. Immediately a child, maybe 12/13, raced from behind me, under my legs and sat down. I said that I had got up specifically to offer my seat to the pregnant woman and his mother, who was sat at the back, said, ''Ignore her, sit down.''
I explained why I had stood up to the mother and the boy but it was no use. Lots of: 'My boy can sit wherever he wants. My boy is a human being with rights'' nonsense. I said a few rather mean things to the mother and was not being unreasonable in doing so. Didn't change the situation but made the pregnant woman laugh and me feel so much better.
Rude, entitled people...I'm sure they're aware that they are making you have to stand, OP.