A statement in the name of a Sinai-based al-Qaida-inspired group has claimed responsibility for four bombings targeting police in the Egyptian capital that killed six people.
The claim in the name of Ansar Beit al-Maqdis, or the Champions of Jerusalem, was posted late on Friday on two main militant websites that have previously carried statements by the group and other al-Qaida allies.
It said the most powerful blast, an attack on Cairo's security headquarters that killed four, was a car bomb that was detonated by remote control.
It also claimed responsibility for three other attacks across the city, two of which killed one person each.
The group, based primarily in the restive Sinai Peninsula, claimed it would carry out more attacks in the future.
A statement said: "We tell our dear nation that these attacks were only the first drops of rain, so wait for what is coming up."
It warned "our people in Egypt to stay away from the police and security headquarters because we suffer a lot when we try to avoid inflicting harm to the Muslims".
Meanwhile, a new bomb went off in the early hours of Saturday next to a police training institute in eastern Cairo, causing no casualties but infuriating already tense residents.
The spokesman for Egypt's Interior Ministry Hani Abdel-Latif told The Associated Press that the blast took place at 7am in the busy district of Ein Shams, but damaged only the institute's walls.
Ahmed Mahmoud, an engineering student living close to the site of the attack, said that the blast shook his building and caused a brief power cut and, when he went down to inspect, he saw a wrecked car and a damaged electricity box.
He saw police cordoning off the area and ambulances rushing in. The police institute is metres away from a metro station.
The bombing came as Egypt marks the third anniversary of the 2011 uprising that toppled long-time autocratic president Hosni Mubarak.
Morsi supporters as well as military loyalists plan rival rallies. It has sparked fears of an intensification of violence following the ouster of Islamist President Mohammed Morsi in a military coup on July 3, which followed millions-strong protests calling for him to leave office.
Mr Mahmoud said that angry residents vowed to attack any Islamist rallies that go their area. "People were saying they will carry arms and kill all Muslim Brothers who dare to pass from this place," he added.
Since Morsi's overthrow, media and security officials regularly report clashes between Muslim Brotherhood protesters and residents of the neighbourhoods through which they march.