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We report on the gamma-ray activity of the blazar Mrk 501 during the first480 days of Fermi operation. We find that the average LAT gamma-ray spectrum ofMrk 501 can be well described by a single power-law function with a photonindex of 1.78 +/- 0.03. While we observe relatively mild flux variations withthe Fermi-LAT (within less than a factor of 2), we detect remarkable spectralvariability where the hardest observed spectral index within the LAT energyrange is 1.52 +/- 0.14, and the softest one is 2.51 +/- 0.20. These unexpectedspectral changes do not correlate with the measured flux variations above0.3GeV. In this paper, we also present the first results from the4.5-month-long multifrequency campaign (2009 March 15 - August 1) on Mrk 501,which included the VLBA, Swift, RXTE, MAGIC and VERITAS, the F-GAMMA,GASP-WEBT, and other collaborations and instruments which provided excellenttemporal and energy coverage of the source throughout the entire campaign. Theaverage spectral energy distribution of Mrk 501 is well described by thestandard one-zone synchrotron self-Compton model. In the framework of thismodel, we find that the dominant emission region is characterized by a size <~0.1 pc (comparable within a factor of few to the size of the partially-resolvedVLBA core at 15-43 GHz), and that the total jet power (~10^{44} erg s^{-1})constitutes only a small fraction (~10^{-3}) of the Eddington luminosity. Theenergy distribution of the freshly-accelerated radiating electrons required tofit the time-averaged data has a broken power-law form in the energy range0.3GeV-10TeV, with spectral indices 2.2 and 2.7 below and above the breakenergy of 20GeV. We argue that such a form is consistent with a scenario inwhich the bulk of the energy dissipation within the dominant emission zone ofMrk 501 is due to relativistic, proton-mediated shocks.

Insights Into the High-Energy Gamma-ray Emission of Markarian 501 from Extensive Multifrequency Observations in the Fermi Era