About 20 men, some carrying assault rifles, tore up large plastic tents and burned storage facilities at the site, where tens of thousands of children are due to attend camp sessions, said Ibrahim Elewa, a private guard who was on duty when they struck.
Two days earlier, a previously unknown militant group, "The Free of the Homeland," issued a statement criticizing the camp's organizer, the United Nations Relief and Works Agency (UNRWA), for, "teaching schoolgirls fitness, dancing and immorality."
Fundamentalist Muslims, or Salafis, whose agenda of global or holy war against the West is against Hamas's nationalist goals, have stepped up attacks in the Gaza Strip over the past several months, targeting Hamas security men and offices.
'Attack on the happiness of children' (Photo: AP)
"No doubt in my mind that it is vandalism linked to a certain degree of extremism. It is an attack on the happiness of children," John Ging, UNRWA's director of operations in Gaza, told reporters at the damaged camp.
Taher al-Nono, spokesman for Gaza's Hamas government, condemned the attack "by a group of gunmen" and pledged authorities "will track down the perpetrators."
Last month, Hamas made its own move against what it viewed as immoral conduct, sending police to break up the Gaza Strip's first major hip-hop concert. It said organizers failed to get a permit.
Ging pledged the summer camp, in which boys and girls will be separated in accordance with traditions and values in the religiously conservative territory, would be held as planned.
"The good news for children is and the good news for their parents is UNRWA will not be intimidated by such attacks," Ging said, adding that Gaza's youngsters "deserve to have a smile on their faces."
Hamas wrestled control of the Gaza Strip from Palestinian President Mahmoud Abbas' secular Fatah movement in fighting in 2007.
The Gaza Strip is under an Israeli-led blockade and the West shuns Hamas over its refusal to recognize Israel, renounce violence and accept existing interim Israeli-Palestinian peace deals.