Gradual dispersal

The dispersal of the sit-ins will be gradual, with protesters given several warnings before police move in, senior security officials said.

At first, warnings will be issued and people will be asked to leave. Police will then use water cannons and tear gas to disperse those who refuse to go.

Another security official said, "Violence will not be used unless the protesters get violent."

Morsi's defiant supporters have fortified the protest camps with sandbags and piles of rocks in anticipation of a crackdown.

Thousands were still camped out at the biggest sit-in, near a mosque in northeast Cairo. At entrances to the sprawling site, men with sticks shouted "God is greatest" to keep morale high.

"I have been here for 28 days and will stay until I die as the issue is now about religion not politics. We want Islam, they want liberalism," said protester Ahmed Ramadan, who quit his job in a Red Sea tourist resort to join the camp.

One security source said action against the protesters has been delayed because larger crowds arrived at the protest camps after news broke that a crackdown is imminent.

Army chief General Abdel Fattah al-Sisi, who ousted Morsi, has come under pressure from hard-line military officers to break up the Brotherhood sit-ins, security sources say.

Almost 300 people have been killed in political violence since Morsi's overthrow, including dozens of his supporters shot dead by security forces in two incidents.