  We present measurements of the properties of the central radio source in M87
using Event Horizon Telescope data obtained during the 2017 campaign. We
develop and fit geometric crescent models (asymmetric rings with interior
brightness depressions) using two independent sampling algorithms that consider
distinct representations of the visibility data. We show that the crescent
family of models is statistically preferred over other comparably complex
geometric models that we explore. We calibrate the geometric model parameters
using general relativistic magnetohydrodynamic (GRMHD) models of the emission
region and estimate physical properties of the source. We further fit images
generated from GRMHD models directly to the data. We compare the derived
emission region and black hole parameters from these analyses with those
recovered from reconstructed images. There is a remarkable consistency among
all methods and data sets. We find that >50% of the total flux at arcsecond
scales comes from near the horizon, and that the emission is dramatically
suppressed interior to this region by a factor >10, providing direct evidence
of the predicted shadow of a black hole. Across all methods, we measure a
crescent diameter of 42+/-3 micro-as and constrain its fractional width to be
<0.5. Associating the crescent feature with the emission surrounding the black
hole shadow, we infer an angular gravitational radius of GM/Dc2 = 3.8+/- 0.4
micro-as. Folding in a distance measurement of 16.8(+0.8,-0.7) Mpc gives a
black hole mass of M = 6.5 +/- 0.2(stat) +/-0.7(sys) 10^9 Msun. This
measurement from lensed emission near the event horizon is consistent with the
presence of a central Kerr black hole, as predicted by the general theory of
relativity.
