JERUSALEM - Gaza Strip militants vowed Saturday that an Israeli soldier captured five years ago would not "see the light" until Palestinian prisoners held by Israel were released.

In Israel, some 400 supporters of Sgt. Gilad Schalit gathered at the border crossing where he was seized by gunmen linked to Gaza's ruling Hamas movement on June 25, 2006.

They waved Israeli flags emblazoned with his likeness and demanded the government do more to secure his release. A relative read a letter from Schalit's grandfather faulting the state for failing to bring the 24-year-old home.

"The people involved talk to us from time to time, stroke our heads, but my beloved grandchild Gilad, through no fault of his own, is still rotting away in a Hamas dungeon like a common criminal," Zvi Schalit wrote.

In Jerusalem, Schalit's parents, his brother and his brother's girlfriend chained themselves to one another and to a railing on the side of a small road leading to the prime minister's residence.

"Our family has been held captive for five years now, we'll stay here as long as we have to," the Ynet news website quoted Schalit's father, Noam Schalit, as saying.

Schalit, a tank crewman, was taken captive after militants tunneled under the Israeli border, killed two soldiers at a border post and dragged him bleeding into Gaza. Hamas has allowed no one to visit him and last offered a sign of life in October 2009.

Hamas' threat to continue holding him until its demands are met was delivered in a 39-second video posted Saturday on the group's website.

Hamas wants Israel to release hundreds of Palestinian militants, including the masterminds of attacks that killed dozens of Israelis. Israeli officials have balked, arguing that releasing the men would put more Israelis in danger.

In the northern Gaza town of Jabaliya, families of some of the estimated 7,500 Palestinian prisoners incarcerated in Israeli jails sat with pictures of their imprisoned sons emblazoned on birthday cakes. Number candles were planted on the cakes, signifying the 12 to 22 years the men had spent in Israeli prisons.

The White House and U.N. Secretary-General Ban Ki-moon issued statements calling for Schalit's immediate release.

Schalit holds dual French-Israeli citizenship, and Israeli media reported that the French ambassador to Israel brought Schalit's parents a letter from French President Nicolas Sarkozy assuring Schalit that "France will not abandon you."

Earlier this week, Hamas rebuffed an appeal from the International Committee of the Red Cross to prove Schalit was still alive. Israeli Prime Minister Benjamin Netanyahu retaliated by saying Palestinian prisoners would be stripped of access to higher education and other unspecified privileges.

Palestinian prisoners have reported that Israeli corrections officials have been confiscating cellphones that had been smuggled to them, and that leading Hamas prisoners have been transferred to solitary confinement.

Kadoura Fares, head of the Palestinian prisoners association, said Israel had already severely limited prisoners' visitation rights.

Hamas spokesman Sami Abu Zuhri called the new measures announced by the prime minister "a violation of international law and international humanitarian law" and urged international intervention to block them.