The investigation of parameters of plasma in an interference H-tee plasma switch during extraction of a nanosecond microwave pulse from high-power compressor was carried out using time-resolved fast-framing optical imaging and optical emission spectroscopy measurements.  The microwave compressor represented a resonant cavity connected to an H-plane waveguide tee with a shorted side arm. The plasma discharge in the switch was triggered by a Surelite laser (λ=532nm).  In experiments with optical imaging of the plasma light emission evolution, the cavity was filled with pressurized air at up to 3×10⁵ Pa pressure. It was found that the plasma has the form of filaments with initial length of ~1 mm and with diameters of 0.2-0.6 mm expanding along the RF electric field with the typical velocity ~5×10⁷ cm/sec. These data together with model of plasma resistivity allows estimation of plasma density as ≤2×10¹⁷ cm^-3. In time-resolved optical emission spectroscopy measurements the cavity was filled with helium at pressure of 2×10⁵ Pa. Nanosecond-scale dynamics of plasma density was obtained by analyzing the shape of He I spectral lines, triplet 2p-4d (4471.5 Å) and triplet 2s-3p (3888.65 Å). These experimental data showed an evident correlation between the rise time of the plasma density and the peak power of the microwave output pulse. Numerical simulations of the microwave energy release from the cavity with the appearance of the plasma yielded a good agreement with measured output pulse peak power and waveform.