A string of explosions struck several security targets in the Egyptian capital Friday in ­the highest-profile attack on Egypt’s military-backed government since the ouster last summer of President Mohamed 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 of the city — 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.

Responsibility for the attacks was claimed late Friday by Ansar Bayt al-Maqdis, an extremist group based in the Sinai Peninsula that has stepped up its assaults on security forces since the military coup in July.

A statement released to jihadist forums urged Egyptians to “stay away from the headquarters and police and security stations.”

Also Friday, four protesters demonstrating in support of Morsi in the southern province of Beni Suef were killed in clashes with police.

The military-backed government’s crackdown on opposition groups has set off nationwide protests by Morsi’s Islamist backers and other activists and has 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, an Egypt expert 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 was heard across a wide swath of the capital and leaving a gaping crater. Cairo’s 19th-century Islamic Art Museum, located across the street, was badly damaged.

In its statement, Ansar Bayt al-Maqdis said the bomb was detonated by remote control.

Later, a pro-military private television 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, 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 against him.

Government officials say 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 recent 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 Egypt’s military chief, Gen. Abdel Fatah al-Sissi, who orchestrated the July coup, are hoping that thousands will take to the streets on Saturday to call for Sissi to run in the upcoming presidential election. Sissi has said he will run “at the request of the people.”

The attacks sparked renewed calls for Sissi’s candidacy, as well as an outpouring of rage against the Muslim Brotherhood, which backed Morsi and has been locked in a campaign of protest against the new government.

“They only know blood,” said Nasser Samin, a doorman who works near where the security vehicles were targeted in Cairo’s Dokki district, 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.

The Brotherhood has repeatedly denied involvement in terrorist activities, and there is no evidence it is linked with Ansar Bayt al-Maqdis.

Just hours before the attack, Ansar Bayt al-Maqdis released to online jihadist forums a 21-minute audio message urging police and soldiers to “stop the war on God and the killing of Muslims” or face more violence. The group released a similar statement on the eve of a deadly car bombing outside a police headquarters in the Nile Delta in December.

Friday afternoon, the Interior Ministry released the names of three suspects it said were involved in the truck bombing but did not specify their affiliation.

“There should be a fast and fair investigation, and they have to tell us who did this and why,” said Mohamed Ali, 55, a taxi driver. “Or in three days everybody will forget about it.”

Since July, the government has imprisoned thousands of Brotherhood members and supporters, including nearly all members of Morsi’s former government. It has also cracked down on journalists and liberal activists who have called for an end to military rule.

Federal prosecutors have charged Morsi and other Brotherhood leaders with incitement to murder. But the state has provided little evidence that the Islamist organization, which renounced violence decades ago, orchestrated any of the recent attacks.

Government buildings, popular protest areas and other parts of the capital have been under lockdown by security forces for months. On Friday, army tanks and coiled walls of barbed wire blocked all traffic from the entrances to Tahrir Square, focal point of the 2011 demonstrations and other protests since.

Egyptian state television reported that a security checkpoint just outside the directorate building had been dismantled minutes before the attack and said the attackers “took advantage” of the opening. Riot police outside the security headquarters briefly moved angry spectators away from the damaged building after the blast as ambulances and fire engines pushed through.

“The interior minister said more than once that he will provide security, that he will bring safety to the streets. Where is that?” said Mohammed El-Asmar, 55.

Some of those gathered at the scene of Friday’s blast denounced President Obama as somehow responsible for Egypt’s ongoing violence. Two foreign journalists reported being chased by civilian mobs. A police officer at a nearby station warned foreigners to stay away, “because you are not loved now.”

Later, onlookers roamed freely over the rubble. Some hauled away wire and chunks of concrete. Sitting in the shade of the damaged building, a riot police officer held up a poster of Sissi as clusters of pro-military demonstrators clapped and cheered for the charismatic general.

Lara El Gibaly and Sharaf al-Hourani contributed to this report.