  Context. A large prominence was observed on September 24, 2013, for three
hours (12:12 UT -15:12 UT) with the newly launched (June 2013) Interface Region
Imaging Spectrograph (IRIS), THEMIS (Tenerife), the Hinode Solar Optical
Telescope (SOT), the Solar Dynamic Observatory Atmospheric Imaging Assembly
(SDO/AIA), and the Multichannel Subtractive Double Pass spectrograph (MSDP) in
the Meudon Solar Tower. Aims. The aim of this work is to study the dynamics of
the prominence fine structures in multiple wavelengths to understand their
formation. Methods. The spectrographs IRIS and MSDP provided line profiles with
a high cadence in Mg II and in Halpha lines. Results. The magnetic field is
found to be globally horizontal with a relatively weak field strength (8-15
Gauss). The Ca II movie reveals turbulent-like motion that is not organized in
specific parts of the prominence. On the other hand, the Mg II line profiles
show multiple peaks well separated in wavelength. Each peak corresponds to a
Gaussian profile, and not to a reversed profile as was expected by the present
non-LTE radiative transfer modeling. Conclusions. Turbulent fields on top of
the macroscopic horizontal component of the magnetic field supporting the
prominence give rise to the complex dynamics of the plasma. The plasma with the
high velocities (70 km/s to 100 km/s if we take into account the transverse
velocities) may correspond to condensation of plasma along more or less
horizontal threads of the arch-shape structure visible in 304 A. The steady
flows (5 km/s) would correspond to a more quiescent plasma (cool and
prominence-corona transition region) of the prominence packed into dips in
horizontal magnetic field lines. The very weak secondary peaks in the Mg II
profiles may reflect the turbulent nature of parts of the prominence.
