A group-theoretical property of the ramification filtration

Let Γ(p) be the Galois group of the maximal p-extension of a complete discrete valuation field with a perfect residue field of characteristic p>0. If v0>-1 and Γ(p)(v0) is the ramification subgroup of Γ(p) in the upper numbering, we prove that any closed non-open finitely generated subgroup of the quotient Γ(p)/Γ(p)(v0) is a free prop-group. In particular, this quotient has no torsion and no non-trivial commuting elements