The Muslim Brotherhood has called for a demonstration by Egypt's presidential palace on Wednesday afternoon to "defend legitimacy", its media spokesman Mahmoud Ghozlan has said.

Tens of protesters have set up tents by Egypt's presidential palace in a sit-in against a constitutional decree issued by President Mohamed Mursi and a draft constitution written by an Islamist-dominated assembly.

Opposition forces marched to the Heliopolis palace on Tuesday demanding "the downfall of the regime" and expressing outrage to Mursi's attempt to expand his powers or pass a controversial charter.

Ghozlan explained that the demonstration is to "protect legitimacy, after Tuesday's brutal violations from one segment who imagined they could shake legitimacy or forcibly impose their views."

He added that the popular group was forced to remind the opposition that the Egyptian people are the ones who have elected the current regime.