The Tahrir gathering was staged as a counter-demonstration two days after Islamist rallies exploded into deadly violence, the protest raised the stakes as the country's interim leaders struggled to put together a new government.
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As the crowds grew, wave after wave of military aircraft skimmed over the capital, with one formation leaving behind long trails of smoke in black, white and red – the colors of the Egyptian flag.
"We are on the street to show the world that it was a popular revolution and not a coup that overthrew" Morsi on Wednesday, said a beaming teacher who gave her name as Magda.
Many banners showed the protesters' anger with the United States for what they perceive as its support for Morsi, as well as American media coverage depicting his ouster as a coup.
"America shame on you! This is a revolution, not a coup!" read one, echoing a chant heard in Tahrir, again and again. Others carried portraits of army Chief of Staff General Abdel Fattah al-Sisi.
President Barack Obama insisted overnight that the United States was "not aligned" with any political party or group in Egypt following Morsi's ouster.
"The future path of Egypt can only be determined by the Egyptian people," the White House quoted him as saying.
The Tamarod movement, which engineered the June 30 rallies that culminated in Morsi's overthrow, had led calls for people to gather at Tahrir and Ittihadiya presidential palace to "complete the revolution".
The anti-Morsi crowd swelled in the iconic square as people poured in from side-streets, some unfurling a giant national flag emblazoned with the words "Go away" – a slogan used widely on June 30.
There were similar scenes in Alexandria on the Mediterranean and in other major cities across the Arab world's most populous country.
Their Islamist rivals staged their own huge demonstrations in Cairo, where police armed with Kalashnikov assault rifles watched over the pro-Morsi demonstrators.
Carrying pictures of the deposed president, the Islamists erected barricades and set up checkpoints across the capital, where tens of thousands of them blocked the main road to the international airport.
Political UnrestSocial Democratic lawyer Ziaad Bahaa el-Din is likely to be appointed interim prime minister of Egypt under a deal emerging among the country's new political forces, a presidential spokesman said on Sunday.
He also said liberal politician Mohamed ElBaradei, whose initial nomination for the post angered a key Islamist party, would probably be appointed interim deputy president instead.
Bahaa el-Din, 48, an commercial lawyer with a doctorate in banking law from the London School of Economics, was head of Egypt's investment authority in the late years of ex-President Hosni Mubarak's rule during a period of economic liberalisation, but resigned before the former autocrat was toppled.
Al-Arabiya television said Bahaa el-Din had asked for time to consider the offer.
The ultra-conservative Salafist Nour party, which objected to the proposed appointment of ElBaradei as interim prime minister, said it was studying the proposal.
One of the founders of the Social Democrat party and a strong critic of ousted Islamist President Mohamed Mursi's rule, Bahaa el-Din argued in favour of Egypt concluding a $4.8 billion loan deal with the International Monetary Fund, which remains stalled because Mursi refused to implement it.
He was elected to Egypt's first parliament after the uprising that overthrew Mubarak in 2011 but the legislature was dissolved last year when the constitutional court ruled that the election law was flawed.
His appointment was suggested as a compromise by the "Tamarud - Rebel!" youth protest movement which mobilised millions of demonstrators to demand Mursi's removal and which has gained strong influence among the new authorities.
"Now the revolution is on its right path, and we tell the Egyptian people we will continue until we get our freedom," said Tamarud co-leader Mahmoud Badr. "We will back Dr. ElBaradei as much as we can with all our might, but whoever commits a mistake, we'll be there to monitor."
The Salafi Islamists, who backed Morsi's overthrow, are continuing to hold out against ElBaradei's appointment, officials close to the talks told AFP and after Sunday's round of talks, a senior Salafi politician said his Nour party would not accept ElBaradei.
"Our position is simple. There are two reasons to reject ElBaradei: we need a technocratic economic figure; and we need to end polarization on the street," said Nader Bakkar.
"We can't talk of national reconciliation and then make Morsi's most ardent opponent prime minister."
Morsi, who has been in custody since Wednesday, had issued a defiant call for his supporters to defend his "legitimacy" as Egypt's first freely elected president, in a recorded speech released shortly after his ouster.
Violence that killed at least 37 people following Friday's Islamist rallies came despite talk of peaceful protests, with Cairo and second city Alexandria the hardest hit.
The bloodletting continued, with gunmen on Saturday killing a Coptic Christian priest in the Sinai Peninsula and militants on Sunday blowing up a gas pipeline to Jordan as Islamists fired on the security forces.
Also, Islamist gunmen attacked a military checkpoint Sunday, in the north of the Sinai Peninsula, killing a soldier, security officials said. According to reports, the attack took place near the town of El-Arish, where Islamist terrorists stormed the headquarters of the governorate of Northern Sinai Friday evening.
Russia's President Vladimir Putin warned the stand-off threatened to degenerate into a civil war.
"Syria is already in the grips of a civil war, unfortunately enough, and Egypt is moving in that direction," news agencies quoted him as saying.
AP, Reuters contributed to this report
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