SU Ursae Majoris the prototype of a subgroup of dwarf novae that display superhumps in super- outbursts has been observed during an international campaign dedicated to the observation of the first day of outburst of dwarf novae during 1986 February. After the start of a brightening was reported, the star was monitored by IUE, ground-based photometry and spectrophotometry, IR-photometry, and radio observations by VLA. However, it did not undergo a normal outburst or a superoutburst. We observed a short and low-amplitude flare, consisting of two phases. The first phase lasted 22 hr, during which the system brightened in the UV and optical wavelengths by factors of 2 and 3.5, respectively; then, at long wavelengths it declined to its initial value, while in the UV the system remained at mid-brightness. In the second phase the system brightened again mainly at long wavelengths and reached half the intensity of the primary brightening. The spectral lines varied but remained in emission during the entire flare, which indicated that no transformation to the optically thick state occurred. The variations of fluxes of the UV emission lines are correlated with changes in the continuum, while the optical lines, observed with higher time resolution, show great variability in relative intensities and equivalent widths. The Balmer line ratios indicate higher densities and temperatures of the disk after the primary brightening, when a drop in luminosity at longer wavelengths was observed. Furthermore, multichannel photometry obtained during the first phase of the flare displayed coherent variations, probably modulated with half of the orbital period or several percent less. The study of the flux distribution during the flare confirmed that it is inconsistent with the simple steady state model of accretion disks, described by a single power law, probably because of the presence of a hot source contributing significantly around 2000 Å. The conclusion is drawn that the observed flare may be consistent with the mass transfer instability model of outburst of dwarf novae. The primary brightening could be interpreted as a burst of mass transfer striking the accretion disk. The enhanced bright spot gave rise to the light modulation. The consecutive drop and secondary brightening at long wavelengths could have resulted from the dissolution of the bright spot and disk shrinking due to the accretion of low angular momentum material into the disk and its successive restoration.