  We prove the scale invariant Harnack inequality and regularity properties for
harmonic functions with respect to an isotropic unimodal L\'{e}vy process with
the characteristic exponent $\psi$ satisfying some scaling condition. We show
sharp estimates of the potential measure and capacity of balls, and further,
under the assumption of that $\psi$ satisfies the lower scaling condition,
sharp estimates of the potential kernel of the underlying process. This allow
us to establish the Krylov-Safonov type estimate, which is the key ingredient
in the approach of Bass and Levin, that we follow.
