Egypt's government has vowed to strengthen its crackdown on Muslim Brotherhood protesters, with the Interior Ministry saying on state television Thursday that security forces will use live ammunition to counter attacks against themselves or state buildings, hours after demonstrators torched two government facilities in Giza.

The Muslim Brotherhood, remaining defiant, said earlier in the day that it would not back down in the wake of Wednesday's violence that left more than 500 dead, calling for supporters to "bring down this military coup."

State television footage showed firefighters evacuating employees from the larger of the two offices in Giza, Cairo's twin city on the west bank of the Nile River, while it was being attacked. Police arrested several protesters.

Witnesses told Reuters that hundreds of Muslim Brotherhood supporters, carrying pictures of former President Mohammed Morsi and of those killed Wednesday, also marched in Alexandria.

"We will push until we bring down this military coup," Muslim Brotherhood spokesman Gehad El-Haddad wrote on Twitter, according to Reuters.

Egypt's government, in a statement, called the demonstrations "terrorist acts" and a "criminal plan to demolish the pillars of the Egyptian state," according to Reuters.

Meanwhile, successive attacks on Coptic Christian churches continued for a second day, according to Egypt's official news agency and human rights advocates.

Egypt's MENA agency said Morsi supporters set fire to the Prince Tadros church in the province of Fayoum, nearly 50 miles southwest of Cairo. The same province witnessed similar attacks on at least three churches in different villages on Wednesday.

El-Haddad said early Thursday that the Muslim Brotherhood would remain "non-violent" in their demonstrations, but later took a different stance, saying that the group has been having a difficult time trying to persuade its members to be peaceful in the wake of Wednesday's bloodshed.

"After the blows and arrests and killings that we are facing emotions are too high to be guided by anyone,'" he told Reuters. "It's not about Morsi anymore. Are we going to accept a new military tyranny in Egypt or not?'"

El-Haddad added that Muslim Brotherhood currently can not account for the whereabouts of several of its leaders, calling it a "very strong blow," to the group.

On Thursday, President Barack Obama told reporters that the U.S. is canceling a joint military exercise with Egypt amid the violence, saying "our traditional operations can not be continued as usual as people are being killed in the streets.”

The U.S. has not declared Morsi's ouster a coup, a move that would require the Obama administration to suspend $1.3 billion in annual military aid.

The death toll in Wednesday's crackdown, which stands at 638, according to the latest Health Ministry figures, made it by far the deadliest day since the 2011 popular uprising that toppled longtime ruler and autocrat Hosni Mubarak. The Health Ministry said Thursday that 4,200 people were wounded.

The casualties were mostly in Cairo, where police in riot gear bulldozed two protest camps that had been the flashpoint of growing unrest following Morsi's July 3 ouster.

Mohammed Fathallah, the ministry spokesman, said that blood-soaked bodies lined up at a mosque-turned-morgue in Nasr City were not included in the official figure. Reuters counted 228 bodies at the mosque.

Inside the building, the names the dead were scribbled on white sheets covering the bodies, some of which were charred, and a list with 265 names was plastered on the wall. Posters of Morsi were piled up on in one corner as the heat made the stench from the corpses almost unbearable.

Many people in the mosque complained that authorities were preventing them from obtaining permits to bury their dead, although the Muslim Brotherhood announced that several funerals had been held for identified victims on Thursday. Fathallah denied the allegations of permits denied.

Cairo, a city of some 18 million people, was uncharacteristically quiet Thursday, with only a fraction of its usually hectic traffic and many stores and government offices shuttered. Many people hunkered down at home for fear of more violence. Banks and the stock market were closed.

Wednesday's violence prompted Interim President Adly Mansour to declare a month-long state of emergency and impose a 7 p.m.-to-6 a.m. curfew in the city, along with Alexandria, and 12 other provinces, ordering the armed forces to support the police in efforts to restore law and order and protect state facilities.

Backed by helicopters, police on Wednesday fired tear gas and used armored bulldozers to plow into the barricades at the two protest camps on opposite ends of Cairo. Morsi's supporters had been camped out since before he was ousted by a July 3 coup that followed days of mass protests by millions of Egyptians demanding that he step down.

The smaller camp — near Cairo University in Giza — was cleared of protesters relatively quickly, but it took about 12 hours for police to take control of the main sit-in site near the Rabaah al-Adawiya Mosque in Nasr City that has served as the epicenter of the pro-Morsi campaign and had drawn chanting throngs of men, women and children only days earlier.

After the police moved on the camps, street battles broke out in Cairo and other cities across Egypt. Government buildings and police stations were attacked, roads were blocked, Interior Minister Mohammed Ibrahim said.

At one point, protesters trapped a police Humvee on an overpass near the Nasr City camp and pushed it off, according to images posted on social networking sites that showed an injured policeman on the ground below, near a pool of blood and the overturned vehicle.

Three journalists were among the dead: Mick Deane, 61, a cameraman for British broadcaster Sky News; Habiba Ahmed Abd Elaziz, 26, a reporter for the Gulf News, a state-backed newspaper in the United Arab Emirates; and Ahmed Abdel Gawad, who wrote for Egypt's state-run newspaper Al Akhbar. Deane and Elaziz were shot to death, their employers said, while the Egyptian Press Syndicate, a journalists' union, said it had no information on how Gawad was killed.

Interim Prime Minister Hazem el-Beblawi said in a televised address to the nation that it was a "difficult day" and that he regretted the bloodshed, but offered no apologies for moving against the protesters, saying they were given ample warnings to leave and he had tried foreign mediation efforts. El-Beblawi added that the government could not indefinitely tolerate a challenge to authority that the 6-week-old protests represented.

"We want to see a civilian state in Egypt, not a military state and not a religious state," he said.

Despite the curfew, sporadic clashes continued in Cairo through the evening.

By early afternoon, dozens of Morsi supporters were blocking a main road near the site of the Nasr City camp, disrupting traffic.

In the city of Assiut, south of Cairo, a police station was hit by two mortar shells Wednesday night fired by suspected Morsi supporters, according to officers there who spoke on condition of anonymity because they were not authorized to talk to reporters.

As the fighting intensified Wednesday, Mohamed ElBaradei abruptly resigned as Egypt's interim vice president. In a letter sent to Mansour, ElBaradei cited "decisions I do not agree with" regarding the government's crackdown.

"It has become difficult for me to continue bearing responsibility for decisions that I do not agree with and whose consequences I fear," ElBaradei wrote. "I cannot bear the responsibility for one drop of blood.''

The National Salvation Front, the main opposition group that he headed during Morsi's year in office, said it regretted his departure and complained that it was not consulted in his decision to step down. Tamarod, the youth group behind the mass anti-Morsi protests that preceded the coup, said ElBaradei was dodging his responsibility at a time when his services were needed.

On Thursday, Reuters reported that Turkish Prime Minister Recep Tayyip Erdogan had called for the United Nations Security Council to convene quickly to discuss the crisis.

Speaking at a press conference in Turkey's capital, Ankara, Erdogan said "Those who remain silent in the face of this massacre are as guilty as those who carried it out."

Secretary of State John Kerry said the violence in Egypt is deplorable and is a serious blow to reconciliation efforts, and that it runs counter to Egyptians' aspirations for peace.

Morsi, Egypt's first freely elected president, had just completed one year in office when he was toppled. He has largely been held incommunicado, but was visited by European Union Foreign Policy Chief Catherine Ashton and an African delegation. Ashton reported that Morsi is well and has access to television and newspapers.

Several bids by the United States, the European Union and Gulf Arab states to reconcile the two sides in Egypt in an inclusive political process have failed, with the Brotherhood insisting that Morsi must first be freed along with several of the group's leaders who have been detained in connection with incitement of violence.

The Associated Press contributed to this report.