CAIRO — A string of explosions struck several security targets in the Egyptian capital on Friday in the highest-profile attack on Egypt’s military-backed government since the ouster last summer of President Mohammed Morsi.
The most powerful blast, outside Cairo’s police headquarters, killed four people and injured more than 50, the Health Ministry said. Three smaller explosions in other districts — one near a police station, another targeting a line of security vehicles, and a third near a movie theater — killed two others, state media said.
Also Friday, four protesters demonstrating in support of Morsi in the southern province of Beni Suef were killed in clashes with police.
The coup last July and the government’s subsequent crackdown on opposition groups have set off nationwide protests by Morsi’s Islamist backers and other activists and ignited a string of deadly attacks on military and police targets across the country.
Friday’s bombing of the Cairo Security Directorate ‘‘is a sign that this low-level insurgency against the Egyptian regime is likely to intensify, and it is increasingly expanding into major urban centers,’’ said Shadi Hamid, a specialist on Egypt and fellow at the Brookings Institution’s Saban Center. ‘‘Cairo is no longer immune to these kinds of attacks.’’
The Interior Ministry said a suicide bomber drove a pickup truck laden with explosives into a barricade outside the security building at 6:30 a.m., causing a thunderous blast that left a gaping crater. Cairo’s 19th-century Islamic Art Museum, across the street, was badly damaged.
Later, a pro-military private channel aired two video clips from a closed-circuit camera that showed a white truck pulling up to the security directorate shortly before the blast. In the first clip, a black car pulls up just as the white truck parks, and a person appears to move to the black car, which then drives away. In the second clip, police officers appear to inspect the truck briefly, before moving back inside the directorate gate seconds before the blast.
The timing of the attack, at the start of Egypt’s weekend, seemed designed to minimize casualties. But the reverberations of the boom, which sent showers of glass across city blocks and tore metal doors and shutters off their hinges, could be heard for miles.
The explosion came on the eve of planned celebrations to mark the anniversary of the start of the country’s 2011 uprising, which toppled longtime strongman Hosni Mubarak. Morsi’s supporters have urged their allies to use the anniversary as a day of protest aimed at reversing the coup.
Government officials said commemorations will take place despite what they describe as the nation’s ongoing war on terrorism. On Friday, military helicopters flew over parts of the city.
The attacks are ‘‘meant to instill fear in the people. But I think that people will be more defiant,’’ Interior Minister Mohamed Ibrahim told reporters at the scene of the blast. ‘‘Our entire plan for the celebration on January 25th still applies. People should not be afraid.’’
But Ramadan Hamza, 33, standing in the rubble, said: ‘‘There is nothing to celebrate. This is chaos, not a revolution.’’
President Adly Mansour, in a statement, compared the current struggle to the crackdown on an Islamist insurgency by Mubarak in the 1990s. ‘‘The Egyptian state, which previously defeated terrorism in the nineties, will uproot it once again, and will show neither pity nor mercy to those who abandoned their nation and moved away from the true teachings of our peaceful religion,’’ the statement said.
Supporters of military chief Abdel-Fattah el-Sissi, who orchestrated the July coup, are hoping that thousands will take to the streets on Saturday to call for Sissi to run in presidential elections. Sissi said he will run ‘‘at the request of the people.’’
The attacks sparked renewed calls for Sissi’s candidacy and an outpouring of rage against the Islamist Muslim Brotherhood, which backed Morsi and is locked in protest against the new government.
‘‘They only know blood,’’ Nasser Samin, a doorman who works near where the security vehicles were targeted in Cairo’s Dokki district, said, referring to the Brotherhood. ‘‘So we must face blood with blood. If Sissi runs, we will elect him.’’
Hundreds gathered outside the ravaged building after the explosion, some chanting, ‘‘The people want the execution of the Muslim Brotherhood!’’ Others carried posters of Sissi and blared nationalist songs.
Since the coup, Egypt’s security forces have cast the Brotherhood, which came to power through Egypt’s first democratic elections in 2012, as a bloodthirsty organization bent on destroying the country. Late last month, the government formally designated the group a terrorist organization.
But the Brotherhood has repeatedly denied involvement in terrorist activities and has condemned the attacks on security forces. Ansar Bayt al-Maqdis, an extremist group based in Egypt’s lawless Sinai Peninsula, has claimed responsibility for most of the attacks.
The Interior Ministry released the names of three suspects it said were involved in the truck bombing but did not specify their affiliation.
Ansar Bayt al-Maqdis did not claim responsibility for Friday’s blasts but the group released a message urging police and soldiers to ‘‘stop the war on God and the killing of Muslims’’ or face more attacks.