Context : The radio quasar 3C 454.3 underwent an exceptional optical outburst lasting more than 1 year and culminating in spring 2005 .
The maximum brightness detected was R = 12.0 , which represents the most luminous quasar state thus far observed ( M _ { B } \sim - 31.4 ) .

Aims : In order to follow the emission behaviour of the source in detail , a large multiwavelength campaign was organized by the Whole Earth Blazar Telescope ( WEBT ) .

Methods : Continuous optical , near-IR and radio monitoring was performed in several bands .
ToO pointings by the Chandra and INTEGRAL satellites provided additional information at high energies in May 2005 .

Results : The historical radio and optical light curves show different behaviours .
Until about 2001.0 only moderate variability was present in the optical regime , while prominent and long-lasting radio outbursts were visible at the various radio frequencies , with higher-frequency variations preceding the lower-frequency ones .
After that date , the optical activity increased and the radio flux is less variable .
This suggests that the optical and radio emissions come from two separate and misaligned jet regions , with the inner optical one acquiring a smaller viewing angle during the 2004–2005 outburst .
Moreover , the colour-index behaviour ( generally redder-when-brighter ) during the outburst suggests the presence of a luminous accretion disc .
A huge mm outburst followed the optical one , peaking in June–July 2005 .
The high-frequency ( 37–43 GHz ) radio flux started to increase in early 2005 and reached a maximum at the end of our observing period ( end of September 2005 ) .
VLBA observations at 43 GHz during the summer confirm the brightening of the radio core and show an increasing polarization .
An exceptionally bright X-ray state was detected in May 2005 , corresponding to the rising mm flux and suggesting an inverse-Compton nature of the hard X-ray spectrum .

Conclusions : A further multifrequency monitoring effort is needed to follow the next phases of this unprecedented event .