Egypt's transition to democracy has been plunged into turmoil after its Supreme Constitutional Court ruled to dissolve the Islamist-led parliament.
The court in Cairo ruled on Thursday that the vote - the first free and fair poll in decades - was unconstitutional and a third of the seats were "illegitimate". It is calling for fresh elections.
Egypt's Muslim Brotherhood warned that the ruling, just two days before an election to replace ousted leader Hosni Mubarak, has put the country's fragile democratic gains under threat.
Islamists who gained most from Mubarak's overthrow decried what they called a "coup" by an army-led establishment still full of Mubarak-era officials. They said the street movement that spurred last year's uprising would not let it pass, Reuters reports.
The parliamentary vote earlier this year had swept long repressed Islamists into a commanding position in the legislature.
The Muslim Brotherhood's Freedom and Justice party won 46% of the vote - a feat the brotherhood had aimed to repeat with their candidate in Saturday and Sunday's presidential vote.
Those parliamentary gains will now be put back up for grabs in a new election.
In a further setback, the court ruled that Mubarak's last prime minister, Ahmed Shafik, could stay in the presidential race against the Brotherhood's Mohamed Morsy in the election on 16-17 June.
American Secretary of State Hillary Clinton said the United States expected Egypt's military authorities to fully transfer power to a democratically elected civilian government.
The Muslim Brotherhood said the court rulings indicated Egypt was heading into "very difficult days that might be more dangerous than the last days of Mubarak's rule".
Outside the court, protesters chanted "Down, down with military rule" and hurled stones at troops lined up in a security cordon. A few hundred also gathered in Tahrir Square in central Cairo.
For 16 months since Mubarak was toppled after 30 years in office, a transition overseen by generals has been beset by political bickering, protests and often bloodshed.
But many Egyptians had taken some reassurance from the calm conduct of the parliamentary election and the prospect of a presidential poll even though the process of writing a new constitution to define the president's powers is in deadlock.
Now even those gains are being plunged into doubt, although the army said the presidential poll would go ahead on time.