While I support the right to choose, I also support people's right to protest and demonstrate. If you want to make a counterdemonstration, that is one thing. Actively trying to stop what is people's constitutional right just plays into their hands. When you attack people (as this call to action seems to insinuate), you only hurt your cause and make people feel more self-righteous about their position.

I think provoking people to think would be far more effective than trying to stop them from marching. You will also come across better in the media. There are a lot of good reasons to uphold the right to choose. You should rely on the power of your arguments rather than compromise your cause through shout downs.