  In this work, we present the first spectroscopic measurements of conversion
electrons originating from the decay of metastable gaseous $^\mathrm{83m}$Kr
with the Karlsruhe Tritium Neutrino (KATRIN) experiment. The results obtained
in this calibration measurement represent a major commissioning milestone for
the upcoming direct neutrino mass measurement with KATRIN. The successful
campaign demonstrates the functionalities of the full KATRIN beamline. The
KATRIN main spectrometer's excellent energy resolution of ~ 1 eV made it
possible to determine the narrow K-32 and L$_3$-32 conversion electron line
widths with an unprecedented precision of ~ 1 %.
