The invention relates to an intrusion detection process for detecting an intrusion within a space, in which at least two acoustic or electromagnetic signals are emitted at different frequencies (f.sub.1, f.sub.2, . . . f.sub.n) inside said space; at a point inside said space, a signal is received that is the combination of the signals emitted and reflected in said space; the characteristics of the demodulated signals of different frequencies are compared to establish the presence or absence of an intrusion; an intrusion signal is generated when the presence of an intruder is confirmed. The invention also proposes an intrusion detection device implementing the process according to the invention.