Initial results of extracted power from a moderately modulated intense relativistic electron beam are reported. The modulation circuit is operated as a relativistic klystron oscillator frequency locked to an external oscillator. A 40% modulated electron beam has yielded more than 1 GW of rms power to a vacuum load and also radiated through an antenna. The ratio of the rf power to the intense electron beam power is approximately 20%.PACS numbers: 84.40.Fe, 07.57.Hm, 41.60.-m A common milestone for all researchers working on high power microwave (HPM) sources has been to extract and radiate rf powers of the order of 1 GW [1][2][3][4][5][6][7]. Many projects also work on providing a long pulse, of order 1 msec, to provide both high power and high energy [1-3]. Many of these efforts have made use of moderate to high voltages (1 MV) at relatively high diode impedances (100 V to 1 kV).We report on an effort to operate at lower impedance and voltage, but with high power and high energy. This has led to complications due to the intense space charge depression of the beam kinetic energy. We began with a Friedman relativistic klystron amplifier (RKA) and converted the system to an injection locked oscillator due to limited power of the external oscillator drive power.This Letter begins with a discussion of the experimental geometry, followed by a discussion of the beam modulation results, and concludes with a brief discussion of extracting and radiating the rf wave.