Martial law is not explicitly mentioned in the Constitution, but the suspension of habeas corpus is mentioned in Article 1, Section 9, and the activation of the militia in time of rebellion or invasion is mentioned in Article 1, Section 8… Martial law should not be confused with military justice…See full article at
http://www.usconstitution.net/consttop_mlaw.html
The following memorandum is fictional but accurately depicts the broad powers enjoyed by presidents to utilize U.S. military forces to address domestic disturbances.
http://www.gunowners.org/prespower.htm
Martial Law in a Nutshell--15 Questions
http://www.prisonplanet.com/articles/november2005/101105martiallaw.htm