Control of the interlayer twist angle in two-dimensional van der Waals (vdW) heterostructures enables one to engineer a quasiperiodic moire superlattice of tunable length scale(1-8). In twisted bilayer graphene, the simple moire super-lattice band description suggests that the electronic band-width can be tuned to be comparable to the vdW interlayer interaction at a 'magic angle'(9), exhibiting strongly correlated behaviour. However, the vdW interlayer interaction can also cause significant structural reconstruction at the interface by favouring interlayer commensurability, which competes with the intralayer lattice distortion(10-16). Here we report atomic-scale reconstruction in twisted bilayer graphene and its effect on the electronic structure. We find a gradual transition from an incommensurate moire structure to an array of commensurate domains with soliton boundaries as we decrease the twist angle across the characteristic crossover angle, theta(c) approximate to 1 degrees. In the solitonic regime (theta < theta(c)) where the atomic and electronic reconstruction become significant, a simple moire band description breaks down and the secondary Dirac bands appear. On applying a transverse electric field, we observe electronic transport along the network of one-dimensional topological channels that surround the alternating triangular gapped domains. Atomic and electronic reconstruction at the vdW interface provide a new pathway to engineer the system with continuous tunability.