The military-backed interim Egyptian government has declared the Muslim Brotherhood a terrorist group after blaming it for a deadly attack on a police HQ earlier this week.
The group, whose candidate Mohammed Morsi won the presidential poll last year before being deposed by the military, had already been outlawed.
Thousands of its supporters have been arrested in a crackdown.
A Muslim Brotherhood leader in exile vowed that protests would continue.
Deputy Prime Minister Hossam Eissa announced the move, which will give the authorities more power to crack down on the Brotherhood.
He said that those who belonged to the group, financed it or promoted its activities would face punishment.