"demonstrate before" is correct and can be used in written English.
It means to show or prove something in front of someone or a group of people. Example: The scientists will demonstrate before the committee to present their findings on the new drug.
Since the Occupy movement has not yet articulated any specific demands, it is difficult to say how long they should be allowed to demonstrate before moving along.
Newcastle supporters are to demonstrate before tomorrow's match with Peterborough in protest at owner Mike Ashley's plans to rename St James Parkk.
"The country wants not only to hold the Copa America, but to also demonstrate before the world that what we want is peace and reconciliation between Colombians," he said.
I was part of a group of doctors and medical educators who recently published in the journal Academic Medicine a set of minimum abilities that every medical student should demonstrate before graduating and caring for elderly patients.
Regarding the Marikana case, the prosecution would have to demonstrate before court that the miners, now murder suspects, had a clear intention to have their fellow colleagues murdered.
"While recruiting some of the world's leading players can lift you into contention for honours, it doesn't necessarily take you all the way, as I hope we will be able to demonstrate before the end of the season as other factors come into play.
"And that's something David has demonstrated before," the executive added, referring to Mr. Bell's tenure as chairman and chief executive at True North Communications before joining Interpublic.
"This hasn't been demonstrated before," insists Angle.
We demonstrated before, that no single round transcription takes place with core enzyme alone (data not shown).
As we have demonstrated before, there was no RVH in our normoxic SERT+ mice.
The feasibility of this approach has been demonstrated before.
