  The production cross sections of the prompt charmed mesons ${\rm D^0}$, ${\rm
D^+}$, ${\rm D^{*+}}$ and ${\rm D_s^+}$ were measured at mid-rapidity in
proton-proton collisions at a centre-of-mass energy $\sqrt{s}=7$ TeV with the
ALICE detector at the Large Hadron Collider (LHC). D mesons were reconstructed
from their decays ${\rm D}^0 \to {\rm K}^-\pi^+$, ${\rm D}^+\to {\rm
K}^-\pi^+\pi^+$, ${\rm D}^{*+} \to {\rm D}^0 \pi^+$, ${\rm D_s^{+}\to
\phi\pi^+\to K^-K^+\pi^+}$, and their charge conjugates. With respect to
previous measurements in the same rapidity region, the coverage in transverse
momentum ($p_{\rm T}$) is extended and the uncertainties are reduced by a
factor of about two. The accuracy on the estimated total $\rm c\overline c$
production cross section is likewise improved. The measured $p_{\rm
T}$-differential cross sections are compared with the results of three
perturbative QCD calculations.
