  We present a determination of the QCD parameter Lambda in the quenched
approximation (n_f=0) and for two flavours (n_f=2) of light dynamical quarks.
The calculations are performed on the lattice using O(a) improved Wilson
fermions and include taking the continuum limit. We find Lambda_{n_f=0} =
259(1)(20) MeV and Lambda_{n_f=2} = 261(17)(26) MeV}, using r_0 = 0.467 fm to
set the scale. Extrapolating our results to five flavours, we obtain for the
running coupling constant at the mass of the Z boson alpha_s(m_Z) =
0.112(1)(2). All numbers refer to the MSbar scheme.
