Reports from Egypt say the interim authorities are planning to start breaking up two sit-ins by supporters of ousted President Mohammed Morsi.
An interior ministry source told the BBC an operation to clear the protest camps would begin shortly before dawn.
However, so far there has not been any police activity at the sit-ins outside the Rabaa al-Adawiya mosque in eastern Cairo and at Nahda Square, to the west.
The Muslim Brotherhood, to which Mr Morsi belongs, has warned of bloodshed.
More than 250 people, most of them Morsi supporters, have already been killed in clashes since the military deposed Egypt's first democratically elected leader on 3 July after mass protests demanding his resignation.
'Prepared for anything'
On Sunday night, a source from the interior ministry said an operation to disperse the two protest camps would begin just before dawn on Monday, and was likely to be a "gradual" process, reports the BBC's Caroline Wyatt in Cairo.
But as the sun rose above the capital, there were no signs of any activity by security forces personnel.
They are thought likely to begin by surrounding both areas, in order to restrict access and stop anyone entering. Shipments of food and water could also be cut off.
Rabaa al-Adawiya
After that, the security forces would step up the use of non-lethal tactics, including tear gas and water cannon, ministry officials told the New York Times.
The camp around the mosque at Rabaa al-Adawiya is surrounded by military sites, and sits on what would normally be a busy dual carriageway.
Protesters have piled sandbags and big rocks around the sit-in, while men wearing motorcycle helmets and carrying sticks have been deployed in anticipation of a raid.
Street vendors have said they have sold hundreds of gas masks.
"We are staying and are psychologically prepared for anything, and have secured the protests areas and their entrances and exits," one of the protesters, Mustafa al-Khateeb, told the Reuters news agency.
Our correspondent says it is not clear whether the warning is a way of again encouraging at least some protesters to leave, though earlier government attempts at persuasion have failed.
For its part, the Muslim Brotherhood has put out its own state of alert, and appears to be asking its followers to stage fresh sit-ins at other main squares in Cairo, she adds.
"We want to send a message to the coup leaders: the Egyptian people insist on continuing their revolution... And the people will insist on turning out in all squares," senior Muslim Brotherhood official Farid Ismail said.
On Sunday, defiant Morsi supporters called for fresh street protests.
"Sisi is a traitor, Sisi is a killer," shouted women who took part in a march through central Cairo, referring to the head of the armed forces, Gen Abdul Fattah al-Sisi.
Our correspondent says the stage is now set for confrontation, and while it is possible that the riot police may try a more nuanced approach, many fear that more blood could be shed.
Over the weekend, the grand imam of al-Azhar, Egypt's top Islamic institution, invited prominent figures to join a meeting on national reconciliation on Monday and discuss his "compromise formula".
The Brotherhood's Freedom and Justice Party (FJP), told the BBC that it was ready for "any kind of dialogue with any intermediary", but questioned Sheikh Ahmed al-Tayyib's impartiality.
The grand imam supported the military intervention to remove Mr Morsi.
Last week, Egypt's interim President, Adly Mansour, blamed the Muslim Brotherhood for the failure of mediation efforts by international diplomats.
