They came in their thousands, bussed in across the desert from thousands of kilometres down the Nile, marching into the square with toddlers on their shoulders, insisting they were prepared to die.
Men in white robes and skullcaps held umbrellas over the heads of women in black niqab full-body veils to shield them from the sun. They carried huge Egyptian flags and chanted "our souls, our blood for Islam".
The ability of deposed president Mohamed Mursi's Muslim Brotherhood to deliver protesters from across Egypt to an extended demonstration in the capital shows that a government crackdown has done little to weaken its organisational might.
The rally near the Rabaa Al Adawiya mosque in northeast Cairo began before Mursi was toppled and is now in its third week. Crowds have stayed in their thousands round the clock, swelling to tens of thousands when the Brotherhood calls special days like Friday's "day of marching on".
Passions have been as hot as the brutal summer sun.
"They killed our martyrs while they were praying!" screamed Soraya Naguib Ahmed, tears down her face visible through the slit of her full-face veil, referring to a clash on Monday when troops killed 53 protesters near a Republican Guard barracks.
"If bullets is what they have to face our people, then they will find us standing in front of the tanks!"
Amer Ali, a member of parliament who spent 13 years as a political prisoner under ousted autocrat Hosni Mubarak, had driven in from Assiut in the Nile valley.
"We're here and we're not leaving," he said at the entrance to the demonstration next to the black jeep Wrangler in which his family would sleep the night. His two-year-old son Mahmoud sat on the car roof holding an Egyptian flag, while his wife sat inside, filming on a tablet, their infant daughter beside her.
"We came with our kids to support legitimacy, democracy, and our civilian president, the first freely elected president in the Arab world."
Mursi's opponents say the Brotherhood demonstrations at the Rabaa Al Adawiya mosque are smaller than the countrywide mass march against him on June 30, attended by millions, which persuaded the army to topple him three days later.
But the feat of keeping supporters' passions burning, and ferrying them from so many disparate locations to the heart of the capital for the ongoing vigil, is a remarkable achievement for an organisation whose senior leaders are now on the run from the authorities. The government says it will not stop peaceful demonstrations.
People squirted water from bottles to cool each other down. Dozens rested in the shade of tents, dozing or reading the Koran, conserving energy during the Ramadan fasting month when Muslims refrain from food and water during daylight.
At a wooden shack constructed on a side street and emblazoned with portraits of Mursi, men prepared vats of rice and sheep meat. Others put the food in plastic bags to prepare to distribute it after sundown, when the tens of thousands gathered will break their fast.
Sahar Mohamed Abdelrahman, 45, a teacher from Menoufiya in the Nile Delta, returned with her adult son and daughter after a one-day break, having slept at the campsite for a week since first arriving last Friday. Her voice was hoarse.
"I want the world to know we are peaceful. We are Muslims and none of us uses violence. We will remain peaceful. I'm not afraid, I only fear God," she said, adding she was ready to die if necessary. "I hope to be a martyr at Rabaa. I wish I could have been one of the martyrs at the Republican Guard. We are peaceful, they are not."
Asma Ismail, 20, a student of Islamic law at Cairo's thousand-year-old al-Azhar University, came with her mother. Her father, jailed frequently under Mubarak, was ill at home.
"I'm staying from now until one of two things happens: either Mursi comes back, or we die for our goal and we live in heaven."