The MAGIC telescope took data of very high energy gamma-ray emission from the blazar Markarian 421 (Mkn 421) between November 2004 and April 2005. We present a combined analysis of data samples recorded under different observational conditions, down to gamma-ray energies of 100 GeV. The flux was found to vary between 0.5 -- 2 Crab units (integrated above 200 GeV), considered a low state when compared to known data. Although the flux varied on a day-by-day basis, no short-term variability was observed, although there is some indication that not all nights are in an equally quiescent state. The results at higher energies were found to be consistent with previous observations. A clear correlation is observed between gamma-rays and X-rays fluxes, whereas no significant correlation between gamma-rays and optical data is seen. The spectral energy distribution between 100 GeV and 3 TeV shows a clear deviation from a power law, more clearly and at lower flux than previous observations at higher energies. The deviation persists after correcting for the effect of attenuation by the extragalactic background light, and most likely is source-inherent. There is a rather clear indication of an inverse Compton peak around 100 GeV. The spectral energy distribution of Mkn 421 can be fitted by a one-zone synchrotron self-compton model suggesting once again a leptonic origin of the very high energy gamma-ray emission from this blazar.

ApJ accepted, 14 pages, 16 figures, 4 tables. updated figures 10, 11, and 12 and one reference