  We construct discrete holomorphic observables in the Ising model at
criticality and show that they have conformally covariant scaling limits (as
mesh of the lattice tends to zero). In the sequel those observables are used to
construct conformally invariant scaling limits of interfaces. Though Ising
model is often cited as a classical example of conformal invariance, it seems
that ours is the first paper where it is actually established.
