Thank you, good pic!
And yes, it does need a special tool:
Jump to tool T Strida workshop tool guide
or use this one:
http://www.parktool.com/product/freewheel-remover-FR-6
Usually the freewheel is tightened very hard!
(I’m going to write a small post for an easy removal procedure asap.)
Be careful when ordering/purchasing these tools, I’ve seen the Shimano tool in USA for over 78 $, while in Spain it was available for just 9,50 Euro (without shipping).
If you’re unable to find it for an acceptable price you might consider to pm me.
Please note also: