CAIRO, July 9 (Reuters) - The Muslim Brotherhood rejected a decree issued by Egypt's interim head of state overnight, which set a timetable for new elections and set up a mechanism to amend the constitution.

Senior Brotherhood politician Essam El-Erian said the decree by interim head of state Adli Mansour was "usurping legislative power" and an act of "someone appointed by the putschists".

Mansour's decree issued overnight sets out a timetable that could lead to parliamentary elections in about six months, to be followed by a new presidential election. The constitution would be amended and again put to public vote in a referendum.

The Brotherhood is demanding the military reinstate Mursi as president and has said it will have nothing to do with the military-backed transition. (Writing by Tom Perry; Editing by Peter Graff)