A population of relativistic runaway electrons in a tokamak can grow rapidly via avalanche mechanism and replace the bulk electron current. The runaway current will then decay slowly in line with dissipation of the stored magnetic energy. This decay follows a marginal stability scenario: the inductive electric field is close to the avalanche threshold at every location where the runaway current density is finite, and the current density vanishes at any point where the field is subcritical. This nonlinear 'Ohm's law' yields a complete picture of the evolving profile of runaway current.