Pro-Morsi protesters, who have been demanding the reinstatement of Egypt's deposed leader for over one month, continued marching in the capital despite warnings that security forces would disperse them imminently.
After spending much of Monday preparing for the security crackdown, some joined marches heading to Rabaa al-Adawiya, one of the main protest sites.
Egyptian security forces are preparing to besiege the supporters of deposed president Mohamed Morsi.
Security sources told Al Jazeera that police would launch action against the protesters early on Monday. However, reports said there was little activity at dawn.
In preparation though, the protesters fortified their camps.
Measures to protect themselves against armoured patrol vehicles and possible force include building concrete and wooden barriers, and buying gas masks, goggles and gloves.
Al Jazeera's Simon McGregor-Wood, reporting from Nasr City, where the protesters have camped out, said that the police action was not expected to be a full-scale assault.
This is a sit-in for all of the sons of Egypt and any attempt to place a siege on it or to impose a slow death by cutting off water or food or electricity is a crime, and anyone responsible will be held accountable
"It will simply be a very comprehensive encirclement of this encampment to try to put the squeeze on," he said.
"They will let people out, but they won't necessarily let them or vital supplies back in."
The decision to take action came after a meeting between the interior minister and his aides, a security source said.
"State security troops will be deployed ... as a start of procedures that will eventually lead to a dispersal," another source said.
The camps are the main flashpoints in the confrontation between the army, which toppled Morsi on July 3, and the ousted president's supporters who demand his reinstatement.
Gamal Heshmat, a senior leader in Morsi's Muslim Brotherhood, said any attempt to besiege the sit-ins would amount to a "crime".
"This is a sit-in for all of the sons of Egypt and any attempt to place a siege on it or to impose a slow death by cutting off water or food or electricity is a crime, and anyone responsible will be held accountable," he said.
"Also any attack or the killing of Egyptians on the basis of political differences as is happening now by the police and army also will be punished, and could ignite all of Egypt."
Thousands rallied on Sunday to demand Morsi's reinstatement, amid last-ditch efforts for reconciliation ahead of the threatened crackdown.
Al-Azhar, Sunni Islam's highest seat of learning, meanwhile, announced plans to resolve the tense political standoff and called for reconciliation talks between the rival sides.
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.
Al Jazeera's D. Parvaz, reporting from near Rabaa al-Adawiya mosque, said a pro-Morsi media centre issued a text message announcing plans to occupy Tahrir Square if the sit-ins are broken up.
"Youth against coup issued a statement after their meeting today stating that after threats of breaking up sit-ins in Rabaa n Nahda and after we took all necessary precautions and measures, we are fully alert and in the case of sit-ins being broken up, ALL youth who are not in sit-ins will move to Occupy Tahrir," said the text message, issued late on Sunday.
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 an assault had begun.