  Let $X$ be a smooth, complete and connected curve and $G$ be a simple and
simply connected algebraic group over $\comp$. We calculate the Picard group of
the moduli stack of quasi-parabolic $G$-bundles and identify the spaces of
sections of its members to the conformal blocs of Tsuchiya, Ueno and Yamada. We
describe the canonical sheaf on these stacks and show that they admit a unique
square root, which we will construct explicitly. Finally we show how the
results on the stacks apply to the coarse moduli spaces and recover (and
extend) the Drezet-Narasimhan theorem. We show moreover that the coarse moduli
spaces of semi-stable $SO_r$-bundles are not locally factorial for $r\geq 7$.
