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We present multicolor optical observations of the nearby (z=0.1685) GRB030329 obtained with the same instrumentation over a time period of 6 hours fora total of an unprecedented 475 quasi-simultaneous BVR observations. Theachromatic steepening in the optical, which occurs at t~0.7 days, providesevidence for a dynamic transition of the source, and can be most readilyexplained by models in which the GRB ejecta are collimated into a jet. Sincethe current state-of-the-art modeling of GRB jets is still flawed withuncertainties, we use these data to critically assess some classes of modelsthat have been proposed in the literature. The data, especially the smoothdecline rate seen in the optical afterglow, are consistent with a model inwhich GRB 030329 was a homogeneous, sharp-edged jet, viewed near its edgeinteracting with a uniform external medium, or viewed near its symmetry axiswith a stratified wind-like external environment. The lack of short timescalefluctuations in the optical afterglow flux down to the 0.5 per cent level putsstringent constraints on possible small scale angular inhomogeneities withinthe jet or fluctuations in the external density

Revealing the Jet Structure of GRB 030329 with High-Resolution Multicolor Photometry