  An electromagnetic response of a single graphene layer to a uniform,
arbitrarily strong electric field $E(t)$ is calculated by solving the kinetic
Boltzmann equation within the relaxation-time approximation. The theory is
valid at low (microwave, terahertz, infrared) frequencies satisfying the
condition $\hbar\omega\lesssim 2E_F$, where $E_F$ is the Fermi energy. We
investigate the saturable absorption and higher harmonics generation effects,
as well as the transmission, reflection and absorption of radiation incident on
the graphene layer, as a function of the frequency and power of the incident
radiation and of the ratio of the radiative to scattering damping rates. We
show that the optical bistability effect, predicted in Phys. Rev. B 90, 125425
(2014) on the basis of a perturbative approach, disappears when the problem is
solved exactly. We show that, under the action of a high-power radiation
($\gtrsim 100$ kW/cm$^2$) both the reflection and absorption coefficients
strongly decrease and the layer becomes transparent.
