Here is Geoffrey Stone, writing on the Chicago law faculty blog:

...[Bush's] assertion of absolute executive authority to detain an American citizen on American soil and to hold him secretly, incommunicado, with no access to a lawyer or to judicial review, merely on the determination of someone in the executive branch that the individual is something called an "enemy combatant." That, in my view, is a "gestapo-like" tactic, plain and simple. It goes wildly beyond anything any other American president has ever done or claimed the power to do.