Egypt begins a two-day runoff presidential vote Saturday pitting former prime minister Ahmed Shafiq against Muslim Brotherhood candidate Mohamed Morsi.

The balloting comes amid political turmoil, following a Supreme Constitutional Court ruling early in the week overturning a law passed by the Islamist-led parliament that barred senior officials from ousted president Hosni Mubarak's government – like Mr. Shafiq – from holding office. The justices – holdovers from the Mubarak era – also cited legal problems with the last round of parliamentary elections and called for the the Islamist-led parliament to be dissolved.

Some leading Islamists accused the country's ruling military council of using the court to stage a de facto coup. But the council said the presidential runoff would go ahead as planned.

Speaking to supporters in Cairo Thursday, Mr. Shafiq called the court ruling “historic” and urged all Egyptians to take part in the polls. But the Muslim Brotherhood says the court ruling indicated that Egypt was heading into “very difficult days that might be more dangerous than the last days of Mubarak's rule.”

Protesters angered by the court ruling took to the streets in Cairo and Alexandria Friday.

Thousands marched to Cairo's Tahrir Square, directing their wrath at Mr. Shafiq, suspicious the former prime minister remains part of a larger plot.

U.S. Secretary of State Hillary Clinton has said Washington is monitoring the political situation in Egypt and expects to see a “full transfer of power to a democratically-elected civilian government.”

U.N. Secretary-General Ban Ki-moon has appealed for a “peaceful and inclusive” vote in Egypt.