Egypt is awaiting official results of a referendum on a new constitution reportedly backed by two-thirds of voters, but which the opposition alleged was riddled with fraud.
The final polling figures will be announced on Tuesday, the head of the electoral commission, Mohamed el-Tanobly, said on Monday.
"The Supreme Elections Committee will announce on Tuesday at 7 P.M. (17:00 GMT) the results of the referendum on the new constitution," judge Samir Abu el-Matti told state radio and TV late on Monday.
Matti also said that the committee, which is led by judges, had spent the last two days investigating opposition and rights' groups accusations of voting fraud.
President Mohamed Morsi's Muslim Brotherhood and state media say an unofficial tally shows 64 percent of ballots backed the new charter after a staggered referendum held December 15 and 22.
If confirmed, the text would be adopted and new legislative elections would have to be called within two months.
But the National Salvation Front opposition coalition claimed numerous instances of polling "fraud and violations" and is demanding the electoral commission investigate before issuing its official figures.
"The referendum is not the end of the road. It is only one battle," the Front said. "We will continue the fight for the Egyptian people."
The challenge suggested no quick end to Egypt's political crisis, which erupted a month ago when Morsi allocated himself near-absolute powers to push through the charter written up by a Muslim Brotherhood-dominated panel.
Fierce protests ensued, including violent clashes on December 5 that killed eight people and wounded hundreds, eventually leading Morsi to give up those powers while defiantly maintaining the referendum.
On Monday, an official statement says Morsi had ordered parliament's upper chamber to convene on Wednesday.
The statement on Monday comes a day after Morsi appointed 90 members to the Muslim Brotherhood-dominated Shura Council. The 90 include at least 30 Muslim Brotherhood members and six minority Christians. The council has a total of 270 members, two-thirds of them elected.
Morsi and his Muslim Brotherhood, testing newfound power after decades of being sidelined by ousted leader Hosni Mubarak, are spearheading the changes.
Against them are ranged the largely urban, liberal, leftwing, Christian and secular supporters of the opposition who feel alienated by Morsi.
They see ambiguities inserted in the charter as opening the way to future strict Islamic law.
Germany is echoing the call for an investigation into the alleged voting fraud, saying the new constitution can only be seen as valid "if the process of its adoption is beyond reproach".
The US, which provides Egypt's powerful military with $1.3bn in aid per year, has kept mostly quiet on the turmoil buffeting its key Middle East ally.
But the Republican chairwoman of the Foreign Affairs Committee in the US House of Representatives, Ileana Ros-Lehtinen, called the vote "a defeat for the Egyptian people" at the hands of "an Islamic dictatorship".
Iran, which is trying to claim the ongoing Arab Spring was inspired by its own 1979 revolution, welcomed the referendum.
It said it promoted "progressive, Islamic and revolutionary goals" in Egypt.
Rating agency Standard and Poor's on Monday downgraded Egypt's long-term credit rating because of "elevated" tensions over its political crisis, and warned it could lower it further.
The country's long-term rating was lowered to "B-" from "B" because the turmoil has "weakened Egypt's institutional framework, and the increasingly polarised political discourse could diminish the effectiveness of policy-making", the agency said.
A $4.8bn loan from the International Monetary Fund was put on hold this month, adding pressure on Egypt's central bank, whose foreign reserves have more than halved since Mubarak's ouster to less than $15bn.