  Fast radio bursts (FRBs) are one of the most tantalizing mysteries of the
radio sky; their progenitors and origins remain unknown and until now no rapid
multiwavelength follow-up of an FRB has been possible. New instrumentation has
decreased the time between observation and discovery from years to seconds, and
enables polarimetry to be performed on FRBs for the first time. We have
discovered an FRB (FRB 140514) in real-time on 14 May, 2014 at 17:14:11.06 UTC
at the Parkes radio telescope and triggered follow-up at other wavelengths
within hours of the event. FRB 140514 was found with a dispersion measure (DM)
of 562.7(6) cm$^{-3}$ pc, giving an upper limit on source redshift of $z
\lesssim 0.5$. FRB 140514 was found to be 21$\pm$7% (3-$\sigma$) circularly
polarized on the leading edge with a 1-$\sigma$ upper limit on linear
polarization $<10%$. We conclude that this polarization is intrinsic to the
FRB. If there was any intrinsic linear polarization, as might be expected from
coherent emission, then it may have been depolarized by Faraday rotation caused
by passing through strong magnetic fields and/or high density environments. FRB
140514 was discovered during a campaign to re-observe known FRB fields, and
lies close to a previous discovery, FRB 110220; based on the difference in DMs
of these bursts and time-on-sky arguments, we attribute the proximity to
sampling bias and conclude that they are distinct objects. Follow-up conducted
by 12 telescopes observing from X-ray to radio wavelengths was unable to
identify a variable multiwavelength counterpart, allowing us to rule out models
in which FRBs originate from nearby ($z < 0.3$) supernovae and long duration
gamma-ray bursts.
