  Aims. We study the optical spectroscopic properties of Type Ia Supernova (SN
Ia) 2004dt, focusing our attention on the early epochs.
  Methods. Observation triggered soon after the SN 2004dt discovery allowed us
to obtain a spectrophotometric coverage from day -10 to almost one year (~353
days) after the B band maximum. Observations carried out on an almost daily
basis allowed us a good sampling of the fast spectroscopic evolution of SN
2004dt in the early stages. To obtain this result, low-resolution, long-slit
spectroscopy was obtained using a number of facilities.
  Results. This supernova, which in some absorption lines of its early spectra
showed the highest degree of polarization ever measured in any SN Ia, has a
complex velocity structure in the outer layers of its ejecta. Unburnt oxygen is
present, moving at velocities as high as ~16,700 km/s, with some
intermediate-mass elements (Mg, Si, Ca) moving equally fast. Modeling of the
spectra based on standard density profiles of the ejecta fails to reproduce the
observed features, whereas enhancing the density of outer layers significantly
improves the fit. Our analysis indicates the presence of clumps of
high-velocity, intermediate-mass elements in the outermost layers, which is
also suggested by the spectropolarimetric data.
