Measurements are reported of 10-100 A electron beam extraction from a vacuum diode and of high-power microwave emissions, of duration <or=1 mu s, from the interaction of the mildly relativistic pulsed electron beam 1< gamma <1.2, with a multimode cavity immersed in a 0.1-0.6 T magnetic field. The temporal behaviour of the diode indicates an interelectrode plasma expanding at 2-4*10⁴ m s^-1. Microwaves in the bands 8.2-12.4 GHz and 26.5-40 GHz have been detected, with a peak power approximately 100 kW near the relativistic electron gyro-resonance. Less than 1 W of incoherent electron cyclotron radiation would be expected. The much higher observed powers are considered to be electron cyclotron maser emissions.