  An intense radio flare on the dMe star AD Leo, observed with the Effelsberg
radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85
GHz is analysed. A lower limit of the brightness temperature of the totally
right handed polarized emission is estimated as T_b ~ 5x10^10 K (with values
T_b > ~3x10^13 K considered to be more probable), which requires a coherent
radio emission process. In the interpretation we favour fundamental plasma
radiation by mildly relativistic electrons trapped in a hot and dense coronal
loop above electron cyclotron maser emission. This leads to densities and
magnetic field strengths in the radio source of n ~ 2x10^11 cm^-3 and B ~ 800
G. Quasi-periodic pulsations during the decay phase of the event suggest a loop
radius of r ~ 7x10^8 cm. A filamentary corona is implied in which the dense
radio source is embedded in hot thin plasma with temperature T >= 2x10^7 K and
density n_ext <= 10^-2 n. Runaway acceleration by sub-Dreicer electric fields
in a magnetic loop is found to supply a sufficient number of energetic
electrons.
