  We consider a magnetic impurity deposited on the surface of a strong
topological insulator and interacting with the surface modes by a Kondo
exchange interaction. Taking into account the warping of the Fermi line of the
surface modes, we derive a mapping to an effective one dimensional model and
show that the impurity is fully screened by the surface electrons except when
the Fermi level lies exactly at the Dirac point. Using an Abrikosov fermion
mean-field theory, we calculate the shape of the electronic density Friedel
oscillation resulting from the presence of the Kondo screening cloud. We
analyze quantitatively the observability of a six-fold symmetry in the Friedel
oscillations for two prototype compounds: Bi$_2$Se$_3$ and Bi$_2$Te$_3$.
