Supporters of Mohammed Morsi have called for protests in the streets, while anti-Morsi activists are staging rallies of their own. Egypt has suffered more than a week of deadly clashes since the president's ouster.
Egypt is bracing for rival demonstrations on Friday. The Muslim Brotherhood, which backs Morsi, has vowed to continue protesting until the ousted president is reinstated.
The Brotherhood "will continue in our peaceful protest until the fall of the military coup and the return of legitimacy," spokesman Ahmed Aref said.
Anti-Morsi protesters have also called on people to stage rallies, including a mass iftar - the breaking of the Muslim fast - in Cairo's Tahrir Square.
Officials say Morsi is still being held in the Republican Guard compound in the capital city, the site of clashes between Islamist protesters and the military Monday that killed 53 supporters of the former Egyptian leader. Four soldiers were killed in the violence, which the military claims was started by terrorists.
Monday's clashes came three days after 35 people were killed across Egypt in fighting between supporters and opponents of Morsi, who has not been seen in public since the army takeover last week that led to the appointment of senior judge Adly Mansour as president.
The Brotherhood says it has been unfairly targeted by the country's interim military leadership. Earlier this week arrest warrants were issued for high ranking officials of the organization, forcing many of them to go into hiding or keep a low profile.
UN, US express detentions concern
In a telephone call to Egyptian Foreign Minister Mohammad Kamal Amr, UN Secretary-General Ban Ki-moon stressed that there should be no retribution against the Brotherhood, or any other political party.
The UN head "expressed deep concern about continued detentions in Egypt and arrest warrants issued against Muslim Brotherhood leaders and others," said UN spokesman Martin Nesirky.
"He reminded the foreign minister of Egypt's international obligations and the need to fully respect the right to freedom of association, speech and due process," said Nesirky. "He made clear that there is no place for retribution or for the exclusion of any major party or community in Egypt."
The White House made clear that there were also anxieties in Washington about the detentions. State Department spokeswoman Jen Psaki said there had been many arrests in recent days "targeting specific groups."
"The arrests we have seen over the past several days targeting specific groups are not in line with the national reconciliation that the interim government and the military say they are pursuing," said Psaki. Washington has been reticent to call the transfer of power a coup. The use of the word would make it no longer legal for the US to provide financial support to the powerful Egyptian military.
dr,rc/lw (Reuters, AFP, AP)