Security sources said the incident occurred in Deir al-Balah, one of many locations in Gaza where hundreds of police were demonstrating on Saturday.
The policemen burnt tires and set up roadblocks to disrupt traffic to protest at their non-payment of their wages by the Hamas government.
More than 160,000 Palestinian Authority employees have received no pay since March, following the suspension of direct Western aid and the refusal of banks to transfer Arab donations to government accounts.
The European Union and United States froze direct aid after the Hamas party took office in March, citing its refusal to recognise Israel, renounce violence and abide by previous agreements.
Most civil servants have been on an open-ended protest strike since September 2.
Samir Abu Aisha, the interim finance minister, said on Thursday that the equivalent of $340 had been paid into each civil servant's accounts.
In other developments on Saturday, a Palestinian shot and wounded by Israeli troops two months ago has died, medical sources said.
Midhat al-Jamal's death brings to 5,404 the number of fatalities since the 2000 outbreak of the Palestinian uprising, the large majority of them Palestinians, according to an AFP count.