  Fast radio bursts (FRBs) are highly dispersed millisecond-duration radio
flashes likely arriving from far outside the Milky Way galaxy. This phenomenon
was discovered at radio frequencies near 1.4 GHz and to date has been observed
in one case at as high as 8 GHz, but not below 700 MHz in spite of significant
searches at low frequencies. Here we report detections of FRBs at radio
frequencies as low as 400 MHz, on the Canadian Hydrogen Intensity Mapping
Experiment (CHIME) using the CHIME/FRB instrument. We present 13 FRBs detected
during a telescope pre-commissioning phase, when our sensitivity and
field-of-view were not yet at design specifications. Emission in multiple
events is seen down to 400 MHz, the lowest radio frequency to which we are
sensitive. The FRBs show a variety of temporal scattering behaviours, with the
majority significantly scattered, and some apparently unscattered to within
measurement uncertainty even at our lowest frequencies. Of the 13 reported
here, one event has the lowest dispersion measure yet reported, implying it is
among the closest yet known, and another has shown multiple repeat bursts, as
described in a companion paper. Our low-scattering events suggest that efforts
to detect FRBs at radio frequencies below 400 MHz will eventually be
successful. The overall scattering properties of our sample suggest that FRBs
as a class are preferentially located in environments that scatter radio waves
more strongly than the diffuse interstellar medium (ISM) in the Milky Way.
