For what it's worth in the UK that would still be trespass, a civil offence. If you had intent to burgle that would be a crime, but if you had no criminal intent it would stay a civil matter.
Well, before the squatting laws were changed, effectively yes. You had to go to court, get a court order, then if they ignored that then bailiffs and possibly police.
Edit: squatting was I believe a special case as squatters gained rights as residents. If you are asked to leave and don't, I believe the police could then be asked to remove you.
Edit: removed incorrect example - thanks seebee