A theft prevention device or system for vehicles permits a vehicle engine to be kept running when the ignition key is switched off and removed. The device includes a relay and momentary switch means, and is activated by engaging the emergency or parking brake, and actuating the momentary switch. In the activated condition, the ignition switch is bypassed, and the relay is energized through a circuit that includes a switch activated by the parking brake. If the parking brake is released without reinserting the key and closing the ignition switch, the ignition system is de-energized. Normal running operation is restored by inserting the ignition key and closing the ignition switch.