  We present the results of a four-month campaign searching for low-frequency
radio transients near the North Celestial Pole with the Low-Frequency Array
(LOFAR), as part of the Multifrequency Snapshot Sky Survey (MSSS). The data
were recorded between 2011 December and 2012 April and comprised 2149 11-minute
snapshots, each covering 175 deg^2. We have found one convincing candidate
astrophysical transient, with a duration of a few minutes and a flux density at
60 MHz of 15-25 Jy. The transient does not repeat and has no obvious optical or
high-energy counterpart, as a result of which its nature is unclear. The
detection of this event implies a transient rate at 60 MHz of 3.9 (+14.7, -3.7)
x 10^-4 day^-1 deg^-2, and a transient surface density of 1.5 x 10^-5 deg^-2,
at a 7.9-Jy limiting flux density and ~10-minute time-scale. The campaign data
were also searched for transients at a range of other time-scales, from 0.5 to
297 min, which allowed us to place a range of limits on transient rates at 60
MHz as a function of observation duration.
