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Hola Ghadeer,
En inglés, demonstration significa (tomado de http://www.macmillandictionary.com/dictionary/british/demonstration)
1) an event that proves a fact
This is a powerful demonstration of what can be achieved with new technology.
2) An occasion when a large group of people protest about something
Angry students held demonstrations in the university square.
demonstration against: There were a number of demonstrations against the new tax.
A massive pro-democracy demonstration soon turned into a riot.
3) an occasion when someone shows people how something works or how to do something.
cookery demonstrations
4) an action that shows how you feel
a demonstration of his love
En español, puedes usar demostración para los casos 1), 3) y 4), pero no para el 2.
Cuando un grupo de gente se reune para protestar por algo, se dice "manifestación".
Espero que te haya servido esta explicación.
Un saludo
