  We consider the inner regions of accretion disks surrounding black holes and
neutron stars and investigate the nonlinear time dependent evolution of
thermal-viscous instabilities. The viscous stress is assumed to be proportional
to the gas pressure with the viscosity parameter formulated as $\alpha
=\min[\alpha_0 (h/r)^n, \alpha_{\max}]$, where $h$ is the local scale height,
$r$ is the distance from the central compact object, and $n$, $\alpha_0$ and
$\alpha_{\max}$ are constants. It is found that the disk is unstable for
$\alpha$ sufficiently sensitive to $h$ ($n \gapprox 1.2$). The instabilities
are globally coherent in the entire unstable region of the disk and, depending
on the viscosity parameters, the time variability of the mass accretion rates
are manifested as periodic or quasi-periodic oscillations. We show that, the
low frequency ($\sim 0.04$~Hz) quasi-periodic oscillations (QPOs) discovered
recently in some of the black hole candidates (Cyg~X-1 and GRO~J0422+32) and a
low mass X-ray binary (Rapid Burster MXB~1730--335) may be explicable by the
thermal-viscous instabilities in accretion disks. The observations of QPOs
place constraints on the viscosity parameters and suggest that $(n,\alpha_0)$
$\sim (1.6,30)$ for the Rapid Burster with a $1.4\,\msun$ neutron star. In the
case of black hole candidates, the dependence of $\alpha$ on $h/r$ is less
steep corresponding to $n \sim 1.2-1.3$ for black holes less than $10\,\msun$.
