  We have combined the wide-area Herschel-ATLAS far-IR survey with
spectroscopic redshifts from GAMA and SDSS to define a sample of 21
low--redshift ($z_{\rm spec} < 0.5$) analogs of submm galaxies (SMGs). These
have been selected because their dust temperatures and total IR luminosities
are similar to those for the classical high-redshift SMG population. As well as
presenting the sample, in this paper we report $^{12}$CO(2-1) and
$^{12}$CO(1-0) observations of 16 low-redshift analogs of SMGs taken with the
IRAM-30m telescope. We have obtained that low-redshift analogs of SMGs
represent a very diverse population, similar to what has been found for
high-redshift SMGs. A large variety in the molecular gas excitation or
$^{12}$CO(2-1)/$^{12}$CO(1-0) line ratio is seen, meaning that extrapolations
from $J \geq 2$ CO lines can result in very uncertain molecular gas mass
determinations. Our sources with $^{12}$CO(1-0) detections follow the dust--gas
correlation found in previous work at different redshifts and luminosities. The
molecular gas mass of low-redshift SMGs has an average value of $M_{\rm H_2}
\sim 1.6 \times 10^{10}\,M_\odot$ and will be consumed in $\sim 100 \, {\rm
Myr}$ . We also find a wide range of molecular gas fractions, with the highest
values being compatible with those found in high-redshift SMGs with
$^{12}$CO(1-0) detections, which are only the most luminous. Low-redshift SMGs
offer a unique opportunity to study the properties of extreme star formation in
a detail not possible at higher redshifts.
