"Forty to fifty youths penetrated the Sanur sector. If the army doesn't manage to dislodge them, we will do it," a police spokesman told reporters on Wednesday.
The protesters, opposed to the Israeli withdrawal from the settlements, forced their way through barricades left by the Israeli army and have occupied the roof of the former British police post of the colony, which unlike the houses of the settlers, was not destroyed after the evacuation, public radio reported.
It is the first time that protesters have reoccupied an evacuated settlement. Some 600 people, the majority protesters opposed to the withdrawal, were evacuated from Sanur on 23 August.
Israel completed its evacuation of Jewish settlers from the Gaza Strip and four small West Bank settlements last week.
The final stage of the withdrawal ended on Monday when Israeli troops left the Gaza Strip, ending 38 years of occupation of the area.