Egypt's Islamist President Mohamed Morsi on Thursday defied calls to curb his sweeping powers, infuriating thousands of protesters who have clashed bloodily with his supporters in recent days.
In an address broadcast live, Morsi vowed to push on with a December 15 referendum on a controversial new constitution, saying "afterwards, there should be no obstacle and everyone must follow its will."
As he was wrapping up his speech, protesters stormed the Cairo villa housing the headquarters of the Muslim Brotherhood which backed him for the presidency, setting it on fire.
"Two hundred thugs went to the headquarters. Security tried to prevent them, but some got through the back door, ransacked it and set it on fire. It is still burning now," Brotherhood spokesman Mahmud Ghozlan said.
Seven people died in clashes between Morsi's Islamist supporters and his mainly secular opponents on Wednesday night in Egypt's worst political crisis since Morsi took office in June. Another 644 people were injured, medical sources said.
The army ordered the square in front of the presidential palace cleared of protesters from both sides, deploying tanks and setting up barbed wire.
In his speech, Morsi said more than 80 people had been arrested.
He railed against the "aggression," implying the opposition protesters were to blame.
"Some attacked cars of the presidency, seriously injuring one of their drivers, who is still in hospital," he said.
"I will never allow anyone to resort to killing and sabotage."
The anti-Morsi camp is furious with the president for assuming sweeping powers two weeks ago and by what it feels was the railroading through by an Islamist-dominated panel of the draft constitution.
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The violence in Cairo recalled scenes seen in the February 2011 uprising that toppled veteran strongman Hosni Mubarak.
Egypt's top Islamic body, Al-Azhar, has called on Morsi to suspend his November 22 decree giving him powers critics say are as sweeping as Mubarak's.
The opposition fears the Islamists are riding roughshod over civil, political and human rights and the rights of women.
"It's the beginning of a religious state," said Sahar Ali, a 39-year-old tour guide and Morsi opponent. "They're trying to turn it into Iran, but we won't let this happen. We got rid of the military -- the Brotherhood is next."
Four of Morsi's advisers have quit over the crisis, the official MENA news agency reported, and the head of state television has also resigned, the independent newspaper Al-Masry al-Youm reported on its website.
The Cairo stock market took a heavy hit from the latest violence, with the EGX-30 index plunging 4.6 percent at the close.
The opposition says it will not stand down until Morsi surrenders his new powers -- which put his decisions beyond judicial review -- and until he cancels the referendum on the draft charter opposed by liberals and Christians.
The United States called for an open and "democratic dialogue" in Egypt.
"The upheaval we are seeing... indicates that dialogue is urgently needed. It needs to be two-way," US Secretary of State Hillary Clinton said in comments echoed by the European Union.
Prominent opposition leader and former UN nuclear watchdog chief Mohamed ElBaradei said Morsi bore "full responsibility" for the violence.
He said the opposition was ready for dialogue but would use any peaceful means to scupper the charter.