We report on measurements performed at low temperatures on a nanoelectromechanical system (NEMS) under (capacitive) parametric pumping. The excitations and detection schemes are purely electrical, and enable in the present experiment the straightforward measurement of forces down to about a femtonewton, for displacements of an Angstrom, using standard room temperature electronics. We demonstrate that a small (linear) force applied on the device can be amplified up to more than a 100 times, while the system is {it truly moving}. We explore the dynamics up to about 50$~$nm deflections for cantilevers about 200$~$nm thick by 3$~$$mu$m long oscillating at a frequency of 7$~$MHz. We present a generic modeling of nonlinear parametric amplification, and give analytic theoretical solutions enabling the fit of experimental results. We finally discuss the practical limits of the technique, with a particular application: the measurement of {it anelastic damping} in the metallic coating of the device with an exceptional resolution of about 0.5$~$%.
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