  We study the static stellar equilibrium configurations ofuncharged and
charged spheres composed by a relativistic polytropic fluid, and compare with
those of spheres composed by a non-relativistic polytropic fluid, the later
case already being studied in a previous work [J. D. Arba\~nil, P. S. Lemos, V.
T. Zanchin, Phys. Rev. D \textbf{88}, 084023 (2013)]. In the relativistic fluid
case, a relativistic polytropic equation of state, $p=\omega\delta^{\gamma}$,
is assumedd. Here, $\delta=\rho-p/(\gamma-1)$, with $\delta$ and $\rho$ being
the rest mass density and the energy density, respectively, and $\omega$ and
$\gamma$ are respectively the polytropic constant and the polytropic exponent.
We assume that the charge density $\rho_e$ is proportional to the energy
density $\rho$, $\rho_e = \alpha\, \rho$, with $\alpha$ being a constant such
that $0\leq |\alpha|\leq 1$. Some properties of the charged spheres such as
mass, total electric charge, radius, redshift, and the speed of sound are
analyzed. The dependence of such properties with the polytropic exponent is
also investigated. In addition, some limits that arise in general relativity,
such as the Chandrasekhar limit, the Oppenheimer-Volkoff limit, the Buchdahl
bound and the Buchdahl-Andr\'easson bound, i.e., the Buchdahl bound for the
electric case, are studied. As in a charged non-relativistic polytropic sphere,
the charged relativistic polytropic sphere with $\gamma\to\infty$ and $\alpha
\to 1$ saturates the Buchdahl-Andr\'easson bound, thus indicating that it
reaches the quasiblack hole configuration. We show by means of numerical
analysis that, as expected, the major differences between the two cases appear
in the high energy density region.
