  We have analysed near-infrared NTT/SofI observations of the starburst cluster
Westerlund 1, which is among the most massive young clusters in the Milky Way.
A comparison of colour-magnitude diagrams with theoretical main-sequence and
pre-main sequence evolutionary tracks yields improved extinction and distance
estimates of A_Ks = 1.13+-0.03 mag and d = 3.55+-0.17 kpc (DM = 12.75+-0.10
mag). The pre-main sequence population is best fit by a Palla & Stahler
isochrone for an age of 3.2 Myr, while the main sequence population is in
agreement with a cluster age of 3 to 5 Myr. An analysis of the structural
parameters of the cluster yields that the half-mass radius of the cluster
population increases towards lower mass, indicative of the presence of mass
segregation. The cluster is clearly elongated with an eccentricity of 0.20 for
stars with masses between 10 and 32 Msun, and 0.15 for stars with masses in the
range 3 to 10 Msun. We derive the slope of the stellar mass function for stars
with masses between 3.4 and 27 Msun. In an annulus with radii between 0.75 and
1.5 pc from the cluster centre, we obtain a slope of Gamma = -1.3. Closer in,
the mass function of Westerlund 1 is shallower with Gamma = -0.6. The
extrapolation of the mass function for stars with masses from 0.08 to 120 Msun
yields an initial total stellar mass of ~52,000 Msun, and a present-day mass of
20,000 to 45,000 Msun (about 10 times the stellar mass of the Orion Nebula
Cluster, and 2 to 4 times the mass of the NGC 3603 young cluster), indicating
that Westerlund 1 is the most massive starburst cluster identified to date in
the Milky Way.
