Egyptian police are set to act against supporters of the deposed president Mohamed Morsi who are gathered in Cairo protest camps, the Reuters news agency has quoted sources as saying.
The security and government sources said police would launch action early on Monday.
"State security troops will be deployed around the sit-ins by dawn as a start of procedures that will eventually lead to a dispersal," a senior security source told Reuters.
The source said that the first step would be to surround the camps.
The Associated Press agency, however, reported that security forces would besiege two protest camps within 24 hours.
Al Jazeera's Simon McGregor-Wood, reporting from the pro-Morsi Cairo suburb of Nasr City, said it was too early to confirm the reports.
"It is worth noting that many people had thought that the government and the military would not act against this place, as long as Eid al-Fitr continues - and that is due to end tonight."
Another security source told Reuters that the decision to take action came after a meeting between the interior minister and his aides.
Thousands rallied on Sunday to demand Morsi's reinstatement, amid last-ditch efforts for reconciliation ahead of the threatened crackdown.
A large convoy of cars carrying pictures of the deposed president beeped their horns as they drove through a neighbourhood in east Cairo.
Hundreds of women marched in central Cairo against army chief Abdel Fattah al-Sisi, shouting: "Sisi is a traitor, Sisi is a killer."
Morsi loyalists have said that nothing short of the deposed president's reinstatement would persuade them to disperse, despite several warnings by the interim leaders that the camps would be dismantled after the Eid al-Fitr holiday.
In a sign of the mounting tensions, a brief overnight power cut at the main sit-in outside the Rabaa al-Adawiya mosque struck panic among the pro-Morsi demonstrators, with some taking to social media to announce that the assault had begun.
The main coalition of Morsi supporters, the Anti-Coup Alliance, said 10 marches would be held in various parts of the capital on Sunday "to defend the electoral legitimacy" of Egypt's first freely elected president.