Gaza Strip militants have vowed an Israeli soldier kidnapped five years ago would not "see the light" until Palestinian prisoners held by Israel were released.
In Israel, some 400 supporters of Sargent 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 Mr 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, Mr 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.
"We are marking five years' anniversary of Gilad's captivity in the Hamas hands and we, as you can see, are also a family in captivity for five years," said Mr Schalit's father, Noam Schalit.
Mr Schalit, a tank crewman, was taken captive after militants tunnelled 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 on the group's website.
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 UN Secretary-General Ban Ki-moon issued statements calling for Mr Schalit's immediate release. Mr Schalit holds dual French-Israeli citizenship, and Israeli media reported that the French ambassador to Israel brought Mr Schalit's parents a letter from French President Nicolas Sarkozy assuring Mr Schalit that "France will not abandon you."