At least five people have been killed in a string of bombings targeting police around Cairo.
The attacks included a suicide car blast that ripped through the city's main police headquarters and wrecked a nearby museum of Islamic artefacts. It is the most significant attack yet in the Egyptian capital at a time of mounting confrontation between Islamists and the military-backed government.
The blasts further hike tensions a day before the third anniversary of Egypt's 2011 uprising that toppled autocrat Hosni Mubarak - when supporters of the military and their Islamist opponents have each vowed rival rallies in the streets to press their cause.
There was no immediate claim of responsibility for today's attacks. Islamic militants have increasingly targeted police and the military since the July 3 coup against Mohammed Morsi and the ensuing crackdown by security forces against his Muslim Brotherhood and other Islamist supporters that has arrested thousands.
Authorities have branded the Brotherhood a terrorist group, accusing it of involvement in the militant violence. The Brotherhood has denied any link. But the branding has helped fuel a wave of popular sentiment against the group and in favour of the military.
Islamists are trying to use tomorrow's anniversary to build momentum in their campaign of protests to "break the coup". Military supporters, in turn, aim to show broad popular support for the government and military chief General Abdel-Fattah el-Sissi, the man who ousted Mr Morsi.
Interior Minister Mohammed Ibrahim, who is in charge of police, called the bombings a "vile terrorist act" aimed at spreading panic ahead of tomorrow's pro-military rallies. "But people will only increasingly insist ... and join the masses in millions", he told reporters at the site of the bombed police headquarters.
The office of interim President Adly Mansour vowed in a statement after the attack that it is determined to "uproot terrorism" and said it could be forced to take "exceptional measures."
On its Twitter account, the Muslim Brotherhood posted a message in English condemning the "cowardly bombings in Cairo, express condolence to families of those killed and demand swift investigation." There was no similar one in Arabic.
Today's violence began around 6:30 am when a suicide car bomber blasted the police HQ in a central square, killing at least four people and sending billows of black smoke into the sky. The Health Ministry said in a statement that four policemen were killed and nearly 50 people wounded.
Several police officers sat on the pavemenet weeping outside the building as ambulances rushed in. A blanket covered a corpse on the ground that officers said was the suicide bomber.
The blast dug a large crater into the pavement, and the street was littered with vehicle parts, shattered glass, bricks and stones. The seven-storey facade of the security headquarters was wrecked, with air conditioning units dangling from broken windows. A nearby courthouse and shops were also damaged.
"Execution for Morsi and his leaders," one man shouted through a megaphone as a crowd gathered around the wreckage chanting against the Brotherhood.
Mr Ibrahim said the attack was carried out by two men in a pick-up truck. The vehicle stopped outside the building, one man got out, apparently when a policeman halted the truck, and then the other man detonated the blast, he said.
The explosion also heavily damaged the renowned Museum of Islamic Art, on the other side of Bab el-Khalq Square.
Windows were blasted out all the way up the facade of the building, which was built in 1881 and recently underwent a multimillion-dollar renovation. The antiquities minister, Mohammed Ibrahim, said artefacts inside were damaged, including a rare collection of Islamic art objects and that the museum will have to be rebuilt.
About two hours later, another bomb struck a police car on patrol near a metro station in the capital's Dokki district, killing one person and wounding eight others.
A third, smaller blast targeted the Talbiya police station about four kilometres (two miles) from the Giza Pyramids but caused no casualties.
The attacks came a day after the country's military and security leaders marked Police Day depicting security forces as national heroes battling terrorism.
An al-Qaida-inspired group called Ansar Beit al-Maqdis, or the Champions of Jerusalem, has claimed responsibility for most of the recent attacks.
A Brotherhood-led coalition had planned protests after Friday prayers across the country as part of their near-daily demonstrations against Mr Morsi's overthrow and the recent vote on the country's rewritten constitution.
Security officials say a bomb planted on a road has hit a convoy of security forces on a busy street in Cairo, killing one person in the fourth bombing in one day.