CAIRO -- Protesters filed into the streets of Cairo after Friday's afternoon prayers as defiant supporters of deposed President Mohamed Morsy promised a "Friday of Anger."
Security forces fired tear gas at a mass of people on a major bridge leading to centrally located Ramses Square. Machine gun fire was heard in central Cairo as a military chopper flew overhead. One witness reported seeing police fire at pro-Morsy protesters exiting a mosque in the western part of the city.
A CNN crew saw that some protesters in the crowd also were armed.
The fear is that the skirmishes foreshadow a violent wave similar to one that broke out Wednesday when hundreds were killed in clashes.
Already Friday, Egyptian emergences services reported nine people had been killed and 44 wounded in clashes, according to state-run Nile TV.
With the military blocking all entrances to Tahrir Square, thousands at Ramses Square, where a heavy cloud of smoke from a building on fire floated into the air, Nile TV reported.
The Muslim Brotherhood has promised huge protests, setting the stage for what could become another catastrophic encounter between security forces and protesters.
"The struggle to overthrow this illegitimate regime is an obligation," the Muslim Brotherhood said on its website Friday, while urging people to protest peacefully.
Military vehicles were deployed Friday across Cairo and Giza, taking up positions in squares and securing important institutions, the state-run EGYNews reported.
The news agency said the military increased checkpoints at all entrances to Cairo to prevent the smuggling of arms to protesters.
A police officer was killed and another was wounded Friday in an attack on a checkpoint in New Cairo, a suburb southeast of the Egyptian capital, state-run newspaper Al-Ahram reported.
Police will use live ammunition to subdue any attack against police facilities, state media said.
Also Friday, at least 20 police officers were wounded when assailants fired on two security cars north of Cairo, according to EGYNews.
Since Wednesday, 64 police officers have been killed, state television reported. It also said 25 police stations and 10 churches have been attacked.
French President Francois Hollande and German Chancellor Angela Merkel called for a meeting next week of European Union foreign ministers to coordinate a response to the violence in Egypt.
Egyptian authorities rejected criticism from U.S. President Barack Obama and other world leaders on Wednesday's ferocious clashes, which left at least 580 people dead after security forces broke up huge sit-ins in Cairo, according to the Health Ministry.
More than 4,000 were injured. Casualties included civilians, police officers and bystanders.
The protesters support Morsy, a former Muslim Brotherhood leader elected president in 2012 whom the military removed July 3. Morsy and some other Brotherhood leaders are under arrest.
On Thursday, state media said Morsy supporters were attacking police stations, hospitals and government buildings outside Cairo. The Interior Ministry said police would use live ammunition against any further attacks.
There also were dozens of reports of attacks, blamed on Morsy supporters, on churches and other Christian facilities across the country .
The "Friday of anger" began with marches from mosques around Cairo, with the aim of converging in Ramses Square.
The most senior Muslim Brotherhood leader still at large, Essam Elerian, said Thursday the protests will continue until Morsy is returned to office.
"They can arrest me and 100 of us, but they can't arrest every honorable citizen in Egypt," Elerian told CNN. "They can't stop this glorious revolution."
Obama cancels joint military exercises
Obama on Thursday announced he had canceled joint U.S.-Egyptian military exercises, which had been scheduled for September.
"Our traditional cooperation cannot continue as usual when civilians are being killed in the streets and rights are being rolled back," the president said.
He called on the Egyptian government to lift a state of emergency decree put in place to limit public gatherings. Addressing the government's opponents, Obama added, "We call on those who are protesting to do so peacefully and condemn the attacks that we've seen by protesters, including on churches."
Churches, schools reported attacked
Dalia Ziada of the Ibn Khaldun Center for Development Studies said Thursday that her group had documented the burning of 29 churches and Coptic facilities across the country.
The Bible Society of Egypt said 15 churches and three Christian schools had been attacked, some set on fire.
Nile TV reported Morsy supporters attacked a church Thursday in Fayoum, southwest of Cairo.
At least 84 people, including Muslim Brotherhood members, have been referred to military prosecutors for charges including murder and the burning of churches, the EGYNews site reported.
Echoes of violence
The violence echoed the upheaval that preceded the fall of Hosni Mubarak from the presidency in 2011. The military removed Mubarak after protests against his authoritarian rule, but not before an estimated 840 people were killed.
The generals yielded power to Morsy after elections, but the new president soon was accused of pursuing an Islamist agenda and excluding other factions from the government. Morsy's supporters say the deposed president wasn't given a fair chance and that the military has returned to its authoritarian practices of the Mubarak era.
The government reinforced the comparison by imposing the monthlong state of emergency, a favored tactic of Mubarak.
Obama warns 'further steps' could be taken
Obama, who has resisted calls to cut off military aid to Egypt and label Morsy's ouster a coup, on Thursday stressed the United States would not support one political faction over another.
"We appreciate the complexity of the situation," the president said. "While Mohamed Morsy was elected president in a democratic election, his government was not inclusive and did not respect the views of all Egyptians. We know that many Egyptians, millions of Egyptians, perhaps even a majority of Egyptians, were calling for a change in course."
But he said he may take unspecified "further steps" because of the government's crackdown.
U.N. High Commissioner for Human Rights Navi Pillay asked for an investigation into the violence.
"The number of people killed or injured, even according to the government's figures, point to an excessive, even extreme, use of force against demonstrators," she said.
Denmark suspended economic aid to the country.
Even predominantly Muslim nations voiced displeasure, with Turkey recalling its ambassador in Egypt in light of the crisis, a Turkish foreign ministry representative said.
CNN's Reza Sayah reported from Cairo; David Simpson wrote from Atlanta. CNN's Hamdi Alkhshali, Saskya Vandoorne, Mohammed Tawfeeq, Ian Lee, Frederik Pleitgen, Laura Smith-Spark, Greg Botelho, Michael Pearson and Holly Yan also contributed to this report.