The Muslim Brotherhood has warned that Egypt faces "dangerous days" ahead, after the Supreme Constitutional Court dissolved the Islamist-dominated parliament.
The court ruled on June 14 that last year's elections for the lower house of parliament, the country's first free elections in decades, were unconstitutional.
The court -- made up of judges appointed by former President Hosni Mubarak before his ouster last year -- also upheld the right of Mubarak's former prime minister, Ahmed Shafiq, to run for president.
The rulings came ahead of the June 16 start of the weekend's runoff presidential election between Shafiq and the Muslim Brotherhood's candidate, Muhammad Morsi.
The decisions effectively push Egypt's democratic transition back to where it was in February 2011, when Mubarak's rule was toppled by an Arab Spring uprising and the military took power.
That's because legislative power has been returned, for now, to the ruling Supreme Council of the Armed Forces, which had handed it over in January 2012 to the newly elected Islamist-dominated parliament.
Activists fear the military council is trying to increase its power, threatening Egypt's fragile democratic gains.
Morsi said any fraud in the June 16-17 presidential runoff vote would result in what he called a "revolution" in Egypt.
"We will continue with our journey and observe closely [the election runoff], and if there is any fraud we already know what the consequences will be -- a revolution against the criminals, a revolution against those who protect the criminals," Morsi said.
"A revolution until the goals of the January 25  revolution are fully achieved."
Morsi's election campaign manager, Ahmed Abdel Moti, said the court ruling backing Safiq's eligibility as a presidential candidate had also stunned Egypt.
"The ruling is a shock to us and to all Egyptians -- not to isolate or exclude [from the presidential election] those [people], one of them a man who was part of the old regime," Moti said.
Clinton: No Turning Back
In Washington, U.S. Secretary of State Hillary Clinton called on Egypt's transitional military rulers to carry through with democratic transition.
"In keeping with the commitments that the Supreme Council of the Armed Forces made to the Egyptian people, we expect to see a full transfer of power to a democratically elected civilian government," Clinton said.
"There can be no going back on the democratic transition called for by the Egyptian people."
U.S. State Department spokeswoman Victoria Nuland said Washington was still studying the Egyptian court's ruling and would not comment on it directly.
But she voiced hope that Egyptians would be allowed to enjoy the gains made from last year's uprising -- "a free, fair, democratic, transparent system of government" and "governance that represents the will of the people, a parliament so elected, a president so elected."
Nuland said those were the standards that both the Egyptian people and the international community wanted.
With reporting by Reuters, AP, AFP, and dpa