Well, I'm hardly in a position to judge the power of various Muslim organizations. But there have certainly been what appear to be very powerful Muslim organizations denouncing terrorism for quite a long time now - all over various news media.
Here's a huge list of "outcries" that certainly appear to mirror the one from the Spanish Islamic Commission, in message and scope:
http://www.unc.edu/~kurzman/terror.htm
I am honestly tired of the "When are Muslims going to denounce terrorism?" argument.