  Within the framework of the massive O($N$) nonlinear sigma model extended to
the next-to-leading order in the chiral counting (for $N=3$ corresponding to
the two(-quark)-flavor Chiral Perturbation Theory), we calculate the
relativistic six-pion scattering amplitude at low energy up to and including
terms $\mathcal{O}(p^4)$. Results for the pion mass, decay constant and the
four-pion amplitude in the case of $N$ (meson) flavors at $\mathcal{O}(p^4)$
are also presented.
