  This Letter reports a measurement of the flux and energy spectrum of electron
antineutrinos from six 2.9~GW$_{th}$ nuclear reactors with six detectors
deployed in two near (effective baselines 512~m and 561~m) and one far
(1,579~m) underground experimental halls in the Daya Bay experiment. Using 217
days of data, 296,721 and 41,589 inverse beta decay (IBD) candidates were
detected in the near and far halls, respectively. The measured IBD yield is
(1.55 $\pm$ 0.04) $\times$ 10$^{-18}$~cm$^2$/GW/day or (5.92 $\pm$ 0.14)
$\times$ 10$^{-43}$~cm$^2$/fission. This flux measurement is consistent with
previous short-baseline reactor antineutrino experiments and is $0.946\pm0.022$
($0.991\pm0.023$) relative to the flux predicted with the Huber+Mueller
(ILL+Vogel) fissile antineutrino model. The measured IBD positron energy
spectrum deviates from both spectral predictions by more than 2$\sigma$ over
the full energy range with a local significance of up to $\sim$4$\sigma$
between 4-6 MeV. A reactor antineutrino spectrum of IBD reactions is extracted
from the measured positron energy spectrum for model-independent predictions.
