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Orla Guerin at the scene of one of the attacks: ''A huge crater was left after the blast''
Five people have been killed and nearly 70 wounded after three blasts in the Egyptian capital that appeared to target the police force.
The attacks began with a powerful car bomb that exploded outside the police headquarters in central Cairo, killing four people and wounding 51.
Within hours, two other blasts occurred elsewhere in the city, killing one person and injuring 15.
The attacks come on the eve of the third anniversary of the 2011 uprising.
The revolution brought about the removal of the country's decades-long ruler Hosni Mubarak.
Analysis
Sally Nabil BBC Arabic, Cairo
The multiple blasts which hit different parts of Cairo appear to be a systematic plan. The attackers managed to hit hard at the heart of the Egyptian capital, close to the police headquarters and the museum of Islamic art.
While the police HQ was the most significant target, another blast near a metro station struck one of the most crowded means of transportation in the city.
Friday is a day off in Egypt, otherwise the scene of this second explosion would have been much worse. A third bomb went off near the Pyramids with no damage or casualties reported.
It is not just about the size of all these blasts but rather about the timing, coming on the eve of the third anniversary of the revolution which toppled Hosni Mubarak.
The minister of interior has repeatedly emphasised that his forces are quite ready to deal with any trouble during this anniversary, but what happened today raises a lot of questions about what the authorities can actually do.
An al-Qaeda-inspired militant group Ansar Beit al-Maqdis (Champions of Jerusalem) has admitted carrying out the attack on the police headquarters.
The group previously claimed responsibility for a car bomb attack on a security building in the northern city of Mansoura in December that killed 16 people and injured more than 100 others.
The authorities blamed Mr Morsi's Muslim Brotherhood for that attack - something the group strongly denied - and declared it a terrorist group shortly afterwards.
Egyptian Interior Minister Muhammad Ibrahim said security is being stepped up around the squares where people are expected to gather to mark Saturday's anniversary.
"We have a plan to secure all of this for the anniversary of the 25 January revolution," he said. "I am telling the people not to be afraid and go down."
Tense timeThe Muslim Brotherhood condemned the attack that hit police headquarters at around 06:30 local time (04:30 GMT).
The group called for an investigation and added they were part of a "peaceful revolution".
The powerful blast was felt across the city and black smoke could be seen rising over the scene.
The interior minister said it appeared to be the work of a suicide bomber.
"A pick-up truck had two passengers inside, stopped outside the security cordon, and the suicide bomber blew himself up," Mr Ibrahim said.
Gunfire was reportedly heard soon after the blast, and more than 30 ambulances raced to the scene.
There was extensive damage to the large concrete building with debris strewn across the street.
Ansar Beit al-Maqdis
- Al-Qaeda-inspired Salafist jihadist group based in Sinai
- Name means Champions of Jerusalem
- Has claimed responsibility for recent deadly attacks around Egypt, and rocket attacks on Israel
- Group said it was behind bombing of police station in Mansoura in Dec 2013 which killed 16 people
Around six police officers sat on the pavement outside and wept, an Associated Press photographer said.
The nearby 19th century Islamic art museum was also damaged. "The building has been destroyed form the outside, but with regards to the antiquities we'll have to wait and see until we can carry out a thorough inspection," said antiquities minister Mohamed Ibrahim.
The BBC's Orla Guerin, reporting from Cairo, says the security directorate is a very significant target and should have been one of the best protected buildings in the city.
As people in Cairo were taking in news of the blast, a second explosion occurred in the Dokki district of the capital.
One person was killed and 15 were wounded in a blast that, according to one report, targeted police vehicles near the metro station.
A short while later, officials said a third device had been set off beside a police station near the famous Giza pyramids. No-one was hurt.
The attacks come at a tense time, with security forces already on heightened alert ahead of the anniversary of the start of the uprising against Mubarak, our correspondent says.
The banned Muslim Brotherhood and its partners are planning demonstrations after Friday prayers across the country.
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They are the latest in a series of regular demonstrations against the overthrow of Islamist President Mohammed Morsi last July - a year after he became Egypt's first democratically-elected leader.
Brotherhood supporters are also angry about recent moves to introduce a new constitution.
Interim Prime Minister Hazem Beblawi, speaking in a BBC interview before the attacks, set out conditions for parliamentary and presidential elections to be held within the next six months.
He said any supporter of the Muslim Brotherhood could take part if "he accepts the new constitution... refuses the use of force and he accepts the idea of a secular government, democratic one, open, no discrimination, no mixing of religion and politics, and accepting human rights".
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