Egypt's Supreme Court has caused widespread alarm by calling for the dissolution of the lower house of parliament and for fresh elections.
Two days before Egyptians choose a new president; it has declared last year's parliamentary vote unconstitutional.
Muslim Brotherhood presidential candidate Mohamed Mursi said the decision "must be respected".
But other political figures have expressed anger amid fears that the military wants to increase its power.
Another senior Muslim Brotherhood politician, Essam Al-Arian, said the ruling on parliament would send Egypt into a "dark tunnel".
The Brotherhood's Freedom and Justice Party won 46% of the vote in the three-month parliamentary poll and Arian warned that the decision would leave the incoming president without a parliament or a constitution.