The restriction of personal liberty is a critical feature in all conflicts, whether they are of an international character or not. With the increased prevalence of non-international armed conflict and the drastic proliferation of non-state armed groups, it is critical to explore whether such groups can legally detain or intern persons during conflict. This article proposes that there exists a power and a legal basis for armed groups to intern persons for imperative security reasons while engaged in armed conflict. It is suggested that this authorisation exists in the frameworks of both international humanitarian law and international human rights law, as it does for states engaged in such conflicts. It is proposed that such power and legal basis are particularly strong for armed groups in control of territory, and can be gleaned from certain customary law claims, treaty law, as well as some case law on international humanitarian law and human rights. Certain case law of the European Court of Human Rights on detention by de facto non-state entities conceivably reflects a change in traditional thinking on ‘legal’ detention by armed groups.