  In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in
capturing the first direct image of the center of the M87 galaxy. The
asymmetric ring morphology and size are consistent with theoretical
expectations for a weakly accreting supermassive black hole of mass
approximately 6.5 x 10^9 M_solar. The EHTC also partnered with several
international facilities in space and on the ground, to arrange an extensive,
quasi-simultaneous multi-wavelength campaign. This Letter presents the results
and analysis of this campaign, as well as the multi-wavelength data as a legacy
data repository. We captured M87 in a historically low state, and the core flux
dominates over HST-1 at high energies, making it possible to combine core flux
constraints with the more spatially precise very long baseline interferometry
data. We present the most complete simultaneous multi-wavelength spectrum of
the active nucleus to date, and discuss the complexity and caveats of combining
data from different spatial scales into one broadband spectrum. We apply two
heuristic, isotropic leptonic single-zone models to provide insight into the
basic source properties, but conclude that a structured jet is necessary to
explain M87's spectrum. We can exclude that the simultaneous gamma-ray emission
is produced via inverse Compton emission in the same region producing the EHT
mm-band emission, and further conclude that the gamma-rays can only be produced
in the inner jets (inward of HST-1) if there are strongly particle-dominated
regions. Direct synchrotron emission from accelerated protons and secondaries
cannot yet be excluded.
