I'm posting the following from http://en.wikipedia.org/wiki/Martial_law and leaving it here for anyone to read:
"United States of America
See also: Suspension clause
The martial law concept in the U.S. is closely tied with the Writ of habeas corpus, which is in essence the right to a hearing on lawful imprisonment, or more broadly, the supervision of law enforcement by the judiciary. The ability to suspend habeas corpus is often equated with martial law. Article 1, Section 9 of the U.S. Constitution states, "The privilege of the Writ of Habeas Corpus shall not be suspended, unless when in Cases of Rebellion or Invasion, the public Safety may require it."
In United States law, martial law is limited by several court decisions that were handed down between the American Civil War and World War II. In 1878, Congress passed the Posse Comitatus Act, which forbids military involvement in domestic law enforcement without congressional approval.
The National Guard is an exception, since unless federalized, they are under the control of state governors. . This has now changed. Public Law 109-364, or the "John Warner Defense Authorization Act of 2007" (H.R.5122), was signed by President Bush on October 17, 2006, and allows the President to declare a "public emergency" and station troops anywhere in America and take control of state-based National Guard units without the consent of the governor or local authorities. Title V, Subtitle B, Part II, Section 525(a) of the JWDAA of 2007 reads "The [military] Secretary [of the Army, Navy or Air Force] concerned may order a member of a reserve component under the Secretary's jurisdiction to active duty...The training or duty ordered to be performed...may include...support of operations or missions undertaken by the member's unit at the request of the President or Secretary of Defense." "