The militants also stole two bulldozers Wednesday and smashed through the border wall between Gaza and Egypt, a Palestinian security official at the border said.
As many as 300 Palestinians crossed into Egypt after the wall was smashed. Brig. Adel Fawzi, director of criminal investigation for North Sinai, said border police were unable to stop the intruders because they had no orders to shoot.
An Egyptian armored vehicle was set on fire and at least three Palestinians were reported injured, one seriously when an Egyptian troop carrier crushed him against a wall, witnesses said.
The gunmen belong to the Al Aqsa Martyrs' Brigades, a violent offshoot of Palestinian leader Mahmoud Abbas' Fatah Party. Fatah-affiliated vigilantes demanding government jobs or trying to get friends out of prison have been responsible for much of the growing anarchy in Gaza, particularly since Israel's pullout in September.
The takeover of government offices by gunmen is an almost daily occurrence in Gaza, reports CBS News correspondent Robert Berger. Palestinian leader Mahmoud Abbas has condemned the anarchy, but seems powerless to stop it.
In other developments:
The latest Gaza incident began late Tuesday, when Palestinian intelligence arrested Alaa al-Hams, an Al Aqsa militant, on suspicion he and his followers kidnapped human rights activist Kate Burton and her parents for two days last week. The Burtons were among 19 foreigners abducted by Fatah gunmen in Gaza in recent months. All have been freed unharmed.
Following al-Hams' detention, his followers fired at the Palestinian security headquarters in the southern town of Rafah where he was being held. Police and gunmen fired in the air, but there were no injuries.
On Wednesday morning, some 40 masked gunmen took over the central election office in Rafah, the local branch of the Palestinian parliament, a local court and another government building. Gunmen were seen on rooftops, inside the buildings and posted at the main doors. Most workers fled the buildings.
A truckload of gunmen then drove to the nearby Rafah border crossing, Gaza's main gate to the world.
Firing in the air, they closed the entrance gate to the crossing compound and told waiting passengers to leave the area. They also set up an impromptu checkpoint at the access road to the crossing, turning away travelers.
Traveler Salima Abu Maghaseeb, 42, said she was angry over the disruption of her plans to travel to Egypt with her daughter for her daughter's wedding later in the week.
"I don't know why the Palestinian Authority is allowing them to do this," said Abu Maghaseeb, who had her documents checked at the impromptu roadblock. "Those people should use their guns ... to protect people and not to come and terrify us. They can go to the border and clash with the Israelis. God only knows what the future holds for Gaza."
The gunmen left the crossing and the public buildings in Rafah after about three hours.
The crossing was handed to Palestinian control, under European supervision, as part of a U.S.-brokered deal with Israel last month. Since then, the crossing was forced to shut down several times during attacks by gunmen.
A spokesman for the European observers, Julio de la Guardia, said the disruptions outside the crossing was an internal Palestinian matter.
"Our functioning at the border crossing has not been disturbed," he said.
Abbas has complained about the growing lawlessness, saying it is harming Palestinian statehood aspirations and economic development. However, he has been unable to impose order, and his failure to keep the gunmen in check is expected to harm Fatah's prospects in a Jan. 25 parliament election.
The tightly run Islamic militant Hamas, whose followers have rarely been involved in vigilante violence, is expected to do well in the vote against the corruption-tainted Fatah.