  Two opposite chiralities of Dirac electrons in a 2D graphene sheet modify the
Friedel oscillations strongly: electrostatic potential around an impurity in
graphene decays much faster than in 2D electron gas. At distances $r$ much
larger than the de Broglie wavelength, it decays as $1/r^3$. Here we show that
a weak uniform magnetic field affects the Friedel oscillations in an anomalous
way. It creates a field-dependent contribution which is {\em dominant} in a
parametrically large spatial interval $p_0^{-1}\lesssim r\lesssim k_Fl^2$,
where $l$ is the magnetic length, $k_F$ is Fermi momentum and
$p_0^{-1}=(k_Fl)^{4/3}/k_F$. Moreover, in this interval, the field-dependent
oscillations do not decay with distance. The effect originates from a
spin-dependent magnetic phase accumulated by the electron propagator. The
obtained phase may give rise to novel interaction effects in transport and
thermodynamic characteristics of graphene and graphene-based heterostructures.
