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Swift triggered on a precursor to the main burst of GRB 061121 (z=1.314),allowing observations to be made from the optical to gamma-ray bands. Manyother telescopes, including Konus-Wind, XMM-Newton, ROTSE and the FaulkesTelescope North, also observed the burst. The gamma-ray, X-ray and UV/opticalemission all showed a peak ~75s after the trigger, although the optical andX-ray afterglow components also appear early on - before, or during, the mainpeak. Spectral evolution was seen throughout the burst, with the promptemission showing a clear positive correlation between brightness and hardness.The Spectral Energy Distribution (SED) of the prompt emission, stretching from1eV up to 1MeV, is very flat, with a peak in the flux density at ~1keV. Theoptical-to-X-ray spectra at this time are better fitted by a broken, ratherthan single, power-law, similar to previous results for X-ray flares. The SEDshows spectral hardening as the afterglow evolves with time. This behaviourmight be a symptom of self-Comptonisation, although circumstellar densitiessimilar to those found in the cores of molecular clouds would be required. Theafterglow also decays too slowly to be accounted for by the standard models.Although the precursor and main emission show different spectral lags, both areconsistent with the lag-luminosity correlation for long bursts. GRB 061121 isthe instantaneously brightest long burst yet detected by Swift. Using acombination of Swift and Konus-Wind data, we estimate an isotropic energy of2.8x10^53 erg over 1keV - 10MeV in the GRB rest frame. A probable jet break isdetected at ~2x10^5s, leading to an estimate of ~10^51 erg for thebeaming-corrected gamma-ray energy.

GRB 061121: Broadband spectral evolution through the prompt and afterglow phases of a bright burst