We study the relaxation of a spin density injected into a two-dimensional electron system with generic spin-orbit interactions. Our model includes the Rashba as well as linear and cubic Dresselhaus terms. We explicitly derive a general spin-charge coupled diffusion equation. Spin diffusion is characterized by just two independent dimensionless parameters, gamma(R) and gamma(D), which control the interplay between different spin-orbit couplings. The real-time representation of the diffuson matrix (Green s function of the diffusion equation) is evaluated analytically. The diffuson describes space-time dynamics of the injected spin distribution. We explicitly study two regimes: The first regime corresponds to negligible spin-charge coupling and is characterized by standard charge diffusion decoupled from the spin dynamics. It is shown that there exist several qualitatively different dynamic behaviors of the spin density, which correspond to various domains in the (gamma(R),gamma(D)) parameter space. We discuss in detail a few interesting phenomena such as an enhancement of the spin-relaxation times, real-space oscillatory dynamics, and anisotropic transport. In the second regime, we include the effects of spin-charge coupling. It is shown that the spin-charge coupling leads to an enhancement of the effective charge diffusion coefficient. We also find that in the case of strong spin-charge coupling, the relaxation rates formally become complex and the spin-charge dynamics is characterized by real-time oscillations. These effects are qualitatively similar to those observed in spin-grating experiments [C. P. Weber , Nature (London) 437, 1330 (2005)].