The microstructure of the nucleus, one of the most studied but least understood cellular organelles, is the subject of much debate. Through the use of particle nanotracking, we detect and quantify the micro-organization as well as the viscoelastic properties of the intranuclear region in single, live, interphase somatic cells. We find that the intranuclear region is much stiffer than the cytoplasm; it is also more elastic than viscous, which reveals that the intranuclear region displays an unexpectedly strong solid-like behavior. The mean shear viscosity and elasticity of the intranuclear region of Swiss 3T3 fibroblasts are 520 Poise (P) and 180 dyn/cm(2), respectively. These measurements determine a lower bound of the propulsive forces (3-15 picoNewton) required for nuclear organelles such as promyelocytic-leukemia bodies to undergo processive transport within the nucleus by overcoming friction forces set by the intranuclear viscosity. Dynamic analysis of the spontaneous movements of nanospheres embedded in the nucleus reveals the presence of putative transient nuclear microdomains of mean size 290+/-50 nm, which are mostly absent in the cytoplasm. The strong elastic character and micro-organization of the intranuclear region revealed by particle nanotracking analysis may help the nucleus to preserve its structural coherence. These studies also highlight the difference between the low interstitial nucleoplasmic viscosity, which controls the transport of nuclear proteins and molecules, and the much higher mesoscale viscosity, which affects the diffusion and directed transport of nuclear organelles and re-organization of interphase chromosomes.