Six policemen were killed and three wounded on Friday when a roadside bomb exploded near a security checkpoint in Cairo, the interior ministry said.
The attack, close to a government building in a middle-class neighbourhood of Cairo, was the latest in a series of security incidents in Egypt often claimed by radical Islamists.
There was no immediate claim of responsibility for the blast in which four civilians were also hurt, security sources said.
The policemen were either in or near their car when the device went off, the sources said.
The ministry said a security cordon had been thrown around the scene of the attack near a mosque in the Giza district of the Egyptian capital.
Eyewitness Ahmed Al-Deeb described a scene of carnage, with dead and dying policemen lying next to wrecked cars. One of the policemen had blast fragments in his chest and two more had lost legs, he told Reuters Television.
Security forces killed three gunmen on Tuesday in a raid on a hideout in southern Egypt used by what they described as an armed wing of the Muslim Brotherhood, which claimed responsibility in September for an assassination attempt on a senior prosecutor.
An Egyptian general was killed by militants on November 4 near his home in North Sinai in an attack for which Islamic State claimed responsibility. He was the second military officer of his rank to be shot dead in as many weeks.
Hundreds of soldiers and police have been killed in an Islamist insurgency led by Islamic State in the Sinai Peninsula.