Demonstrate can be transitive or intransitive. In the example you've given, it is intransitive. 'To demonstrate' (intran.), means to gather with others and take part in a protest or rally of some sort. You can also, however, demonstrate something (transitive).
Jill invented a completely new kind of mouse for the computer. She demonstrated how it works last Friday. It looks very easy to use!
John sells vacuum cleaners. He demonstrates them at the fair, and makes many sales.
The verb 'happened' is intransitive.