A constitutional court ruling on Thursday means that the whole of the lower house of Egypt's parliament will be dissolved and a new election will have to be held, the court's head Farouk Soltan told Reuters by telephone after the ruling was issued.

The court also gave Ahmed Shafik, Hosni Mubarak's last prime minister, green light to continue his bid for Egypt’s presidency, when ruled against a law that would have thrown him out of the race.

"The ruling regarding parliament includes the dissolution of the lower house of parliament in its entirety because the law upon which the elections were held is contrary to rules of the constitution," Soltan said, speaking two days before another election to pick a new president.

Soltan said the ruling was binding on all institutions of state, adding that it would be up to the executive to call for the new election that he said would take place.

A presidential run-off vote between Shafik and the Muslim Brotherhood's Mohamed Morsy takes place on Saturday and Sunday.

Shafik welcomed court rulings that void a parliamentary vote and allow him to contest the country's leadership, saying an "era of political score settling" was over.

A senior Muslim Brotherhood politician said Egypt would enter "a dark tunnel" if the Islamist-dominated parliament was dissolved as required by a ruling from the constitutional court on Thursday.

"If parliament is dissolved, the country will enter a dark tunnel - the coming president will face neither a parliament nor a constitution," Erian told Reuters by telephone. "There is a state of confusion and many questions."

Outside the court by the Nile, protesters threw rocks at hundreds of troops and state security conscripts who were guarding the building, which was sealed off by rolls of barbed wire. Some of the security forces began unloading dozens of boxes of tear gas canisters.

The court threw out a law passed by the Islamist-dominated parliament in April that denied political rights to anyone who held a senior post in government or ruling party in the last decade of Mubarak's rule.

The Brotherhood said it would accept the overturning of the law. "It's a reality now, and we must deal with it as such," said spokesman Mahmoud Ghozlan.

The legal wrangling adds to the suspense around an election that is supposed to seal a transition to democracy after Mubarak was toppled in an Arab Spring uprising last year, but has laid bare deep divisions over how Egypt should be governed.

The law denying political rights to Mubarak's officials had prompted a state election committee to disqualify Shafik from the presidential race, but he was let back in on appeal, pending the court ruling.

The drama is emblematic of the tortuous and messy transition overseen by a council of generals since Mubarak was ousted 16 months ago.

A first-round presidential vote last month pushed more moderate candidates out of the race and the choice now facing 50 million eligible voters reflects a society torn between desire for change after six decades of military rule and anxiety over the damage wrought on Egypt by the subsequent political chaos.

Unrest has simmered on the streets of Egypt's cities throughout the period of military rule, with opponents of the army calling for the removal of "feloul", or Mubarak-era remnants, from politics. The demand is far from unanimous.

"Shafik is a respectable man. We want him as president because we are not ready for Egypt to return to the Dark Ages," said Shafik supporter Sawsan Ali Abdo.

"There is no such thing as 'feloul'. We are all Egyptians. No to the plot seeking to divide Egypt," read one banner in the capital.