Protesters in Egypt have stormed and ransacked the Cairo headquarters of President Mohamed Morsi's Muslim Brotherhood.
A group of protesters was seen Monday smashing windows, setting fires and running off with furniture and files from the building.
The headquarters had been evacuated after earlier violence when demonstrators attacked the building with rocks and firebombs. Brotherhood members and staff inside responded with gunfire.
Activists say at least five people were killed in the violence, which intensified late Sunday after massive protests across Egypt demanding Mr. Morsi's resignation.
Hundreds of protesters remained overnight in Cairo's Tahrir Square to continue to call for action against what they say is a government that is trying to monopolize power and is failing to fix the struggling economy. Many have also voiced anger at Egypt's worsening crime and persistent political and religious violence.
The demonstrations Sunday were the largest since the 2011 revolution that swept former president Hosni Mubarak from power. They were largely peaceful, but clashes in Cairo and in southern Egypt left at least six people dead and hundreds more injured.
The protests Sunday included secular and liberal opponents of Mr. Morsi, who gathered on the first anniversary of his inauguration as Egypt's first freely elected leader. Arabic-language media quoted the Interior Ministry saying the crowds in Cairo and other cities across Egypt totaled as many as 3 million people.
Earlier Sunday in the capital, Mr. Morsi's Islamist backers rallied around a mosque near the presidential palace, vowing to defend him against what they see as opposition efforts to oust a democratically elected leader. Morsi supporters also accuse the opposition of trying to return Egypt to the Mubarak era.
As tensions mounted, the Presidential office issued a statement saying dialogue is the only way out of Egypt's political crisis.
In an interview published Sunday in the British newspaper The Guardian, the president said that if he gave in to the pressure, a new president could face similar opposition demands to quit after a "week or a month."
Street battles linked to the political tension killed at least seven people in the past week, among them an American student stabbed to death while photographing protests in Alexandria.
U.S. Deputy National Security Adviser Ben Rhodes said the United States has boosted security at its embassy and consulates in Egypt. Speaking Sunday, he also said the Obama administration has been in touch with Egyptian government officials and opposition figures to urge them to resolve their differences peacefully.