Protesters seeking to force Egyptian President Mohamed Morsi from office are gearing up for a second day of action, after large crowds thronged the streets of Cairo and cities around the country and marched on the presidential palace.
In the capital, the seat of the powerful Muslim Brotherhood to which Morsi belongs, was set ablaze before people stormed and looted the building, an AFP correspondent there said.
People were seen leaving with petrol bombs, helmets, flak jackets, furniture, televisions and documents.
Some preparing for rallies on Monday spent the night in dozens of tents pitched at Cairo's central Tahrir Square and the palace, positions organisers say they will hold until Morsi resigns.
In fewer numbers, supporters of the Egyptian president came out on Sunday to show their support and defend what they called the president’s "legitimacy".
As anger against Morsi swept the streets, at least six people were killed and more than 600 wounded in clashes between the pro and anti-Morsi groups, the Reuters news agency reported.
The main opposition Tamarod - Arabic for rebellion - movement, which led the demonstrations, gave Morsi a deadline of Tuesday to quit, threatening a campaign of civil disobedience if he stays.
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The number of people who joined in protests on Sunday, the anniversary of Morsi's first year in office, was between 14m and 17m people, the interior ministry told Al Jazeera.
"It is absolutely fair to say that an unprecedented number of Egyptians went to the streets accross the country," said Al Jazeera's Hoda Abdel Hamid, reporting from Cairo.
She added that a statement from Morsi during the protests was not welcomed by many, as it did not offer anything new.
In the message released on Sunday, he said: "I believe we can come together and find a way that builds our country," and he could "engage in national dialogue."
In its statement, Tamarod urged "state institutions including the army, the police and the judiciary to clearly side with the popular will as represented by the crowds."
The group also rejected presidential calls for dialogue, saying: "There is no way to accept any halfway measures.
"There is no alternative other than the peaceful end of power of the Muslim Brotherhood and its representative, Mohamed Morsi."
A few kilometres away from the presidential palace, thousands of Morsi supporters also staged their own sit-in to show support for their president.
"If we are saying that we have a majority, and the opposition are saying that they have a majority, how can they decide?" asked Nader Omran, a spokesman for the Freedom and Justice Party, the political wing of the Muslim Brotherhood.
"What is the other solution for this dilemma, except the ballot box?"
Morsi announced to all of Egypt's people he made mistakes and that he is in the process of fixing these mistakes
Presidential spokesman Omar Amer said Morsi was serious in his repeated calls for national dialogue.
"[Morsi] announced to all of Egypt's people he made mistakes and that he is in the process of fixing these mistakes," Amer told a late-night news conference.
The duelling rallies on Sunday only further highlighted the deepening political polarisation in Egypt.
Morsi supporters are full of praise for his first year in office, insisting that the president has strengthened civilian rule in Egypt and done his best to manage a failing economy.
Many of them dismissed Sunday’s protests as the work of ex-regime figures and “thugs”, fuelled by a hostile media and Western governments.
Anti-government protesters, on the other hand, dismissed Morsi’s first term as a failure and described him as a dictatorial leader. Many accused him of backing the Palestinian group Hamas and other armed groups; one well-dressed man in Tahrir insisted that Morsi planned to cede the Sinai peninsula to Hamas.
But their main complaint was the worsening economy, which has been in free-fall since Morsi took office, with the Egyptian pound losing nearly 20 percent of its value and industry crippled by fuel and electricity shortages.
"He's borrowed money from everyone in the world," said Said Ahmed, referring to $11bn in loans Egypt has received from Qatar, Saudi Arabia and Turkey to prop up the economy. "Who's going to pay for that? Our children."