Egypt's Islamist President Mohamed Morsi has defied calls to curb his sweeping powers or suspend proposed changes to the constitution, infuriating thousands of protesters who have clashed bloodily with his supporters in recent days.
In an address broadcast live on Thursday, Morsi vowed to push on with a December 15 referendum on the 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.
"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," Brotherhood spokesman Mahmud Ghozlan said.
Police outside the three-storey building said it was a limited blaze and that riot officers had pushed demonstrators back.
An AFP correspondent at the scene said there were fierce clashes between hundreds of stick-wielding protesters and police, who fired tear gas.
Seven people died in clashes between Morsi's Islamist supporters and his mainly secular opponents on Wednesday in Egypt's worst political crisis since Morsi took office in June. Another 644 people were injured, medical officials said.
The army on Thursday 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.
"We respect peaceful freedom of speech but I will never allow anyone to resort to killing and sabotage."
Morsi offered to hold dialogue with the opposition and to meet their representatives on Saturday in his offices, but there was no immediate indication of compromise judging by his speech on Thursday.
But Hussein Abdel Ghani, spokesman for the opposition group the National Rescue Front, dismissed Morsi's gesture, saying "the president lost a historic chance to act like a president for all Egypt".
He added: "We will continue to escalate (protests), using peaceful means."
The United States and European Union have called for dialogue to resolve the political crisis in Egypt.
US President Barack Obama expressed "deep concern" Thursday over the events in Egypt, in a call to his counterpart Morsi, the White House said.
Obama also told Morsi that it was "essential for Egyptian leaders across the political spectrum to put aside their differences and come together to agree on a path that will move Egypt forward," the White House said in a statement.
The anti-Morsi camp is furious with Morsi 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.
The violence in Cairo recalls 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.