Hundreds of supporters of President Mohamed Morsi gathered outside Egypt's Supreme Constitutional Court on Sunday, ahead of ruling on the legality of a panel that drafted a controversial new charter.

The Islamists, many wrapped in blankets and carrying posters of Morsi, had spent the night outside the courthouse in a bid to prevent its judges from entering, one protester told AFP.

The court's judges are expected to deliver a verdict on the legality of the Islamist-dominated constituent assembly which drafted the constitution, defying a presidential decree barring any judicial body from dissolving the panel.

Their decision to go ahead with the hearing further piles pressure on Morsi, who has been accused by the opposition of taking on dictatorial powers by issuing the decree that they say places his decisions beyond judicial overview.

The November 22 decree has sparked violent protests and calls for strike and put the Islamist president on a collision course with the judiciary.

Morsi called on Egyptians on Saturday to vote in a December 15 referendum on the controversial draft constitution at the heart of a political crisis, amid mass Islamist rallies in Cairo.

Morsi made the announcement following a ceremony where he received a copy of the charter from the head of the Islamist-dominated constituent assembly, boycotted by liberals and Christians, that adopted it the day before.

Hundreds of thousands of Islamists rallied from early on Saturday in support of Morsi's new expanded powers and the contested charter. The document has taken centre stage in the country's worst political crisis since his election in June.

Crowds flooded the squares and large avenues near Cairo University, led by members of the Muslim Brotherhood, on whose ticket Morsi ran for office, and by hardline Salafists, causing traffic jams in the capital.

On Friday, opponents of the draft constitutions had massed in Tahrir Square, demonstrating the country's widening polarisation that has set largely Islamist forces against secular-leaning opponents.

"We want this phase to end. We want a constitution," one protester said on Saturday. "If people don't like the constitution, let them say so through the ballot boxes."

Others chanted: "The people want the implementation of God's law."

"We are here to support the decisions of Dr Mohamed Morsi; we support him because those decisions were a part of the revolutionary demands," said Hend Abdellateef.

The crisis was sparked when Morsi issued a decree on November 22 giving himself sweeping powers and placing his decisions beyond judicial review. That move provoked mass protests and a judges' strike.

His decree prevented the top legal body the Supreme Constitutional Court from potentially dissolving the Islamist-run constituent assembly, in a ruling it was to make on Sunday on the body's legality.

Amnesty International said the draft "raises concerns about Egypt's commitment to human rights treaties," specifically ignoring "the rights of women (and) restricting freedom of expression in the name of religion."