  We present a search for transient radio sources on time-scales of seconds to
hours at 144 MHz using the LOFAR Two-metre Sky Survey (LoTSS). This search is
conducted by examining short time-scale images derived from the LoTSS data. To
allow imaging of LoTSS on short time-scales, a novel imaging and filtering
strategy is introduced. This includes sky model source subtraction, no cleaning
or primary beam correction, a simple source finder, fast filtering schemes and
source catalogue matching. This new strategy is first tested by injecting
simulated transients, with a range of flux densities and durations, into the
data. We find the limiting sensitivity to be 113 and 6 mJy for 8 second and 1
hour transients respectively. The new imaging and filtering strategies are
applied to 58 fields of the LoTSS survey, corresponding to LoTSS-DR1 (2% of the
survey). One transient source is identified in the 8 second and 2 minute
snapshot images. The source shows one minute duration flare in the 8 hour
observation. Our method puts the most sensitive constraints on/estimates of the
transient surface density at low frequencies at time-scales of seconds to
hours; $<4.0\cdot 10^{-4} \; \text{deg}^{-2}$ at 1 hour at a sensitivity of 6.3
mJy; $5.7\cdot 10^{-7} \; \text{deg}^{-2}$ at 2 minutes at a sensitivity of 30
mJy; and $3.6\cdot 10^{-8} \; \text{deg}^{-2}$ at 8 seconds at a sensitivity of
113 mJy. In the future, we plan to apply the strategies presented in this paper
to all LoTSS data.
