The39thStep I don't agree.
We oppose fascists because of their role in the class struggle. They provide a means by which the ruling class can destroy working class organisation and resistance in times of extreme crisis. The ways we oppose fascists reflect that role - we drive them out of our unions, we deny them the political space to propagandise by refusing to share a platform with them and wherever possible denying them a platform, we make sure they are opposed whenever they try to march or demonstrate, finally, we make it known that they are fascists and not (just?) the patriots or Christians or whatever cover they seek to use.
The various strands of extreme Islamic fundamentalism are unable (and do not seek) to serve the same function in the UK, so they don't present the same threat to the working class. Also, they organise differently and with different goals. Therefore we need to find different ways to oppose them.
What is a march "against extremism" for?
Is it for working with Muslim communities helping them fight the racism discrimination and abuse they face?
Or is it for supporting the government's Prevent programme and getting people to grass on their family and friends for thought crimes?
More importantly, how will such a march be perceived by communities that have faced years of BNP/EDL/NF/BF thugs being escorted through their towns?
This is why "the left" doesn't organise "anti-jihadist" demonstrations. Just as we didn't organise anti-IRA marches during the troubles. In fact we campaigned for Troops Out and in support of the hunger strikers.
The best way for the non-Muslim left in the UK to oppose jihadist terrorism is to provide an alternate pole of attraction through our unions and our parties, by building the fight against racism and fascism, and by opposing bloody imperialist adventures.