Vibrations of telescopes can be successfully corrected in real time using seismometer as an inertial reference. This technique will be needed in adaptive optics systems with polychromatic laser guide stars to remove the tilt component of mechanical origin, it can also be used to compensate wind- induced shake in any telescope. A prototype pendular seismometer is described which is sensitive to pointing errors in the frequency band from few tenths to several tens of Hz. The average pendulum position is maintained by a slow servo system which also damps its resonance. The prototype instrument has a r.m.s. noise of 3 milliarcseconds in the 0 - 25 Hz band, sufficiently low for diffraction-limited imaging with 10 m class telescopes. It was tested on a 1 m telescope, and a good agreement of the seismometer signal with the direct optical measurements of telescope's optical axis fluctuations was found. A frequency response of the seismometer is studied, and an expression for the r.m.s. residual (uncompensated) vibrations is given.