GAZA CITY, Gaza Strip (AP) – Hamas fighters overran one of the rival Fatah movement’s most important security installations in the Gaza Strip on Thursday, and witnesses said the victors dragged vanquished gunmen from the building and executed them in the street.

The capture of the Preventive Security headquarters was a major step forward in Hamas’ attempts to complete its takeover of all of Gaza.

The moderate President Abbas of Fatah, for the first time in five days of fierce fighting, ordered his elite presidential guard to strike back. But his forces were crumbling fast under the onslaught by the better-armed and better-disciplined Islamic fighters.

Fatah officials said seven of their fighters were shot to death in the street outside Preventive Security. A witness, Jihad Abu Ayad, said the men were being killed before their wives and children.

“They are executing them one by one,” Mr. Abu Ayad said. “They are carrying one of them on their shoulders, putting him on a sand dune, turning him around and shooting.”

Some of the Hamas fighters kneeled down outside the building, touching their foreheads to the ground in prayer. Others led Fatah fighters out of the building, some of them shirtless or in their underwear, holding their arms in the air. Several of the Fatah men flinched as the crack of gunfire split the air.

“We are telling our people that the past era has ended and will not return, ” Islam Shahawan, a spokesman for Hamas’ militia, told Hamas radio. “The era of justice and Islamic rule have arrived.”

Sami Abu Zuhri, a Hamas spokesman, heralded what he called “Gaza’s second liberation,” after Israel’s 2005 evacuation of the coastal strip.