  We present results of a multi-line study of the filamentary infrared dark
cloud G351.78-0.54 in the 1.3 and 0.8 mm wavelength bands. The lines of the
three isotopologues of carbon monoxide CO, N$_2$H$^+$, CH$_3$CCH and HNCO were
observed. The aim was to study the general structure of the filamentary cloud,
its fragmentation and physical parameters with the emphasis on properties of
dense clumps in this cloud. Several dense clumps are identified from the
N$_2$H$^+$ (3-2) data, their masses and virial parameters are determined using
the C$^{18}$O (2-1) line. Temperatures of some clumps are estimated from the
CH$_3$CCH and HNCO data. Almost all clumps appear to be gravitationally
unstable. The density estimates obtained from the C$^{18}$O (3-2)/(2-1) and
N$_2$H$^+$ (3-2)/(1-0) intensity ratios are in the range $n \sim (0.3-3)\times
10^5$ cm$^{-2}$. The HNCO emission is detected exclusively toward the first
clump which contains the luminous IR source IRAS 17233-3606, and indicates an
even higher density. It is observed in the outflow, too. The velocity shift of
the higher excitation HNCO lines may indicate a movement of the hot core
relative the surrounding medium. In some clumps there is a velocity shift $\sim
1$ km s$^{-1}$ between N$_2$H$^+$ (3-2) and CO isotopologues. The large widths
of the N$_2$H$^+$ (3-2) line in the clumps indicate an increase of the velocity
dispersion in their dense interiors, which may be related to the star formation
process. The N$_2$H$^+$ abundance drops toward the luminous IR source.
