An explosion rocked a busy square in central Cairo late on Friday. One police officer was killed in the blast.
At least three other people were injured including a senior police officer.
A security official said a bomb was planted inside a traffic post and it later exploded.
But a second official said the bomb was hurled from a bridge over the traffic post.
It was not immediately possible to reconcile the two accounts.
Security forces sealed off the area while looking for more devices.
No group has claimed responsibility for the attack.
Since the army toppled Egypt’s first freely-elected president, Mohamed Mursi of the Muslim Brotherhood, last year, Islamist militants have stepped up attacks on members of the security forces, killing hundreds.
On Tuesday, two bomb attacks wounded three policemen in central Cairo.