  Prestellar cores are self-gravitating dense and cold structures within
molecular clouds where future stars are born. They are expected, at the stage
of transitioning to the protostellar phase, to harbor centrally concentrated
dense (sub)structures that will seed the formation of a new star or the
binary/multiple stellar systems. Characterizing this critical stage of
evolution is key to our understanding of star formation. In this work, we
report the detection of high density (sub)structures on the thousand-au scale
in a sample of dense prestellar cores. Through our recent ALMA observations
towards the Orion molecular cloud, we have found five extremely dense
prestellar cores, which have centrally concentrated regions $\sim$ 2000 au in
size, and several $10^7$ $cm^{-3}$ in average density. Masses of these
centrally dense regions are in the range of 0.30 to 6.89 M$_\odot$. {\it For
the first time}, our higher resolution observations (0.8$'' \sim $ 320 au)
further reveal that one of the cores shows clear signatures of fragmentation;
such individual substructures/fragments have sizes of 800 -1700 au, masses of
0.08 to 0.84 M$_\odot$, densities of $2 - 8\times 10^7$ $cm^{-3}$ and
separations of $\sim 1200$ au. The substructures are massive enough ($\gtrsim
0.1~M_\odot$) to form young stellar objects and are likely examples of the
earliest stage of stellar embryos which can lead to widely ($\sim$ 1200 au)
separated multiple systems.
