The assailants opened fire on the policemen who were posted near the University of Mansura in Egypt's Nile Delta region, before fleeing, one source said.
Since the army's ouster of the country's first freely elected president, Islamist Mohamed Morsi, near-daily attacks targeting security forces have left dozens dead, particularly in the lawless Sinai peninsula.
The interim government installed by the army accuses pro-Morsi Islamist groups of being behind the attacks.
Jihadist movements have claimed many of the attacks, slamming what they call a brutal military coup and have demanded Morsi's return to power.
More than 2,000 members and supporters of Morsi's Muslim Brotherhood have also been arrested since August, including Morsi and its top leadership, who will face trial for inciting the killing of protesters.