Does protesting in the streets ever work?

Does doing mass protest ever work?

On local issues I can see it working. Ie protesting against your local council or your workplace.

But on huge issue like war. Vietnam war you have demostrations after demonstations. Did that stop the war? The US only stopped when they lost the war. The iraq war protest didn't stop the war. In UK the mass protest against the ban on fox hunting didnt change anything.

Live 8 is a mass demostration by itself against the G8 summit. Does it do anything?

when ever street protest are looking like they are having a effect the media down plays what's going on.