Mohamed ElBaradei, founders' representative for the Constitution Party, called on every Egyptian citizen on Friday to participate in today's protests.
"I ask of every Egyptian to participate in today's peaceful protests to save the nation," ElBaradei wrote on his Twitter account.
"Egypt is now experiencing a defining moment," he added.
Political and civil powers on Thursday called for staging demonstrations today to protest against the constitutional declaration decreed by President Mohamed Mursi yesterday.
Commenting on Mursi's decisions, ElBaradei wrote, "Today Mursi blasted the concept of the state and the legitimacy and appointed himself ruler by divine decree. The revolution is aborted until further notice."