CAIRO (Reuters) - Hundreds of thousands of Egyptians flooded into the streets on the first anniversary of Islamist President Mohamed Mursi's inauguration on Sunday to demand that he resign.
Waving national flags and chanting "Get out!", a crowd of more than 200,000 had massed by sunset on Cairo's central Tahrir Square in the biggest demonstration since the 2011 uprising that overthrew Mursi's predecessor, Hosni Mubarak.
"The people want the fall of the regime!" they shouted, echoing the Arab Spring rallying cry that brought down Mubarak - this time yelling it not against an ageing dictator but against the first elected leader in Egypt's 5,000 year recorded history.
Many bellowed their anger at Mursi's Muslim Brotherhood, accused of hijacking the revolution and using electoral victories to monopolise power and push through Islamic law.
Others have been alienated by a deepening economic crisis and worsening personal security, aggravated by a political deadlock over which Mursi has presided.
As the working day ended and 38 Celsius (100 Fahrenheit) heat eased, more protesters converged through the eerily deserted streets of the shuttered city centre, while smaller crowds protested in several other areas of the capital.
The veteran leaders of Egypt's secular, liberal and left-wing opposition, including former chief of the U.N. nuclear watchdog Mohamed ElBaradei and leftist presidential candidate Hamdeen Sabahi, joined protest marches in Cairo.
A Reuters journalist said hundreds of thousands of anti-government protesters marched through the Mediterranean port of Alexandria, Egypt's second city, and a military source reported protests in at least 20 towns around the country.
Mursi, an engineering professor propelled to power by the Muslim Brotherhood, was monitoring events from the heavily guarded Qubba presidential palace, where an official spokesman appealed for the demonstrations to remain peaceful.
"Maintaining the security of Egypt is the common responsibility of everyone," presidential spokesman Ehab Fahmy told a news conference. "Dialogue is the only way to reach mutual understanding and to reach national agreement around the different issues of our homeland."
Security sources said three Brotherhood offices were set on fire by demonstrators in towns in the Nile Delta - the latest in more than a week of sporadic violence in which hundreds have been hurt and several killed, including an American student.
More than 20,000 supporters of Mursi congregated outside a Cairo mosque not far from another suburban presidential palace, where protest organisers planned a sit-in from Sunday evening.
Interviewed by a British newspaper, Mursi voiced his determination to ride out what he sees as an undemocratic attack on his electoral legitimacy. But he also offered to revise the new, Islamist-inspired constitution, saying clauses on religious authority, which fuelled liberal resentment, were not his choice.
He made a similar offer last week, after the head of the army issued a strong call for politicians to compromise. But the opposition dismissed it as too little to late. They hope Mursi will resign in the face of large numbers on the streets.
"We call on Mohamed Mursi, who has completely lost the legitimacy of his power, to quickly respond to the clear will of the people which is plain today in all corners of revolutionary Egypt," the June 30 movement, which organised a nationwide petition demanding his resignation, said in a statement.
Some Egyptians seem to believe the army might force the president's hand, if not to quit then at least to make major concessions to the opposition.
In Cairo, demonstrators stopped to shake hands and take photographs with soldiers guarding key buildings. At least six high-ranking police officers took to the Tahrir Square podium in support of demonstrators, a Reuters witness said.
The armed forces used military helicopters to monitor the protests in Cairo and Alexandria and a military source said chief-of-staff and Defence Minister General Abdel Fattah al-Sisi was following the situation from a special operations room.
Mursi and the Brotherhood hope the protests will fizzle like previous outbursts last December and in January. If they do not, some form of compromise, possibly arbitrated by the army, may be on the cards.
Both sides insist they plan no violence but accuse the other - and agents provocateurs from the old regime - of planning it.
The U.S.-equipped army shows little sign of wanting power but warned last week it may have to step in if deadlocked politicians let violence slip out of control.
U.S. President Barack Obama called for dialogue and warned trouble in the most populous Arab nation could unsettle an already turbulent region. Washington has evacuated non-essential personnel and reinforced security at diplomatic missions.
In an interview with London's Guardian newspaper, Mursi repeated accusations that what he sees as entrenched interests from the Mubarak era are plotting to foil his attempt to govern. But he dismissed the demands that he give up and resign.
If that became the norm, he said, "well, there will be people or opponents opposing the new president too, and a week or a month later, they will ask him to step down".
Liberal leaders say nearly half the voting population - 22 million people - has signed a petition calling for new elections, although there is no obvious challenger to Mursi.
The opposition, fractious and defeated in a series of ballots last year, hope that by putting millions on the streets they can force Mursi to relent and hand over to a technocrat administration that can organise new elections.
Religious authorities have warned of "civil war". The army insists it will respect the "will of the people", though the two sides have opposing views of what that means.
Islamists interpret that to mean army support for their election victories. Opponents believe that the army may heed the popular will as expressed on the streets, as it did in early 2011 when the generals decided Mubarak's time was up.
Having staged shows of force earlier this month, the Brotherhood has not called on supporters to go out on Sunday.
Among the Islamists in Cairo, Ahmed Hosny, 37, said: "I came here to say, 'We are with you Mursi, with the legitimate order and against the thugs'.
"This is our revolution and no one will take it from us."
At Tahrir Square, banners ranged from "The Revolution Goes On", "Out, Out Like Mubarak" to "Obama Backs Terrorism" - a reference to liberal anger at U.S. support for Mursi's legitimacy and criticism of protests as bad for the economy.
"I am here to bring down Mursi and the Brotherhood," said Ahmed Ali al-Badri, a feed merchant in a white robe. "Just look at this country. It's gone backwards for 20 years. There's no diesel, gasoline, electricity. Life is just too expensive."
The army, half a million strong and financed by Washington since it backed a peace treaty with Israel three decades ago, says it has deployed to protect key installations.
Among these is the Suez Canal. Cities along the waterway vital to global trade are bastions of anti-government sentiment. A bomb killed a protester in Port Said on Friday. A police general was gunned down in Sinai, close to the Israeli border.
There are some similarities with Turkey, where an Islamist prime minister with a strong electoral mandate was confronted in the streets by angry secularists this month. But Egypt is much poorer, its economy is crumbling rather than booming and its new democracy was born in a revolution just two years ago.
For many Egyptians, all the turmoil since 2011 has just made life harder. Standing by his lonely barrow at an eerily quiet downtown Cairo street market, 23-year-old Zeeka was afraid more violence was coming.
"We're not for one side or the other," he said. "What's happening now in Egypt is shameful. There is no work, thugs are everywhere ... I won't go out to any protest.
"It's nothing to do with me. I'm the tomato guy."
(Reporting by Asma Alsharif, Alexander Dziadosz, Shaimaa Fayed, Maggie Fick, Alastair Macdonald, Shadia Nasralla, Tom Perry, Paul Taylor and Yasmine Saleh in Cairo, Yusri Mohamed in Ismailia and Abdelrahman Youssef in Alexandria; Writing by Alastair Macdonald and Paul Taylor; Editing by Peter Graff)