Events marking the 36th annual Prisoners' Day were held throughout the West Bank and Gaza on Saturday following the death of a Palestinian inmate at the Eshel Prison in south Israel.
Raed Abu Hamad, 31, was found dead in his cell late Friday, and his lawyer said an autopsy is being planned. Hamad, who was sentenced in 2005 to 10 years in prison for a nationalistically-motivated attempted murder, was considered a dangerous and violent prisoner.
The Palestinian Prisoner Affairs minister, Issa Qaraqi, said Saturday that the prisoner had been in solitary confinement for the last 18 months and that this worsened his condition.
Hamad's lawyer, Tareq Barghouth, said his client required psychological help but none was provided during his five years in detention. Israel said he was on medication.
Israel has nearly 6,700 Palestinians in custody, and their release is a key Palestinian demand.
The Israel Prison Service and Israel Police have launched an investigation into Hamad's death.
Thousands of Palestinians, including relatives of prisoners held in Israel, participated in the main Prisoners' Day event in Ramallah. Minister Qaraqi told those on hand that the Palestinian Authority (PA) would continue working towards releasing all Palestinian prisoners from Israeli jails.
"I see hope and the meaning of victory in you and in the photos you carry of your loved ones," he said.
Senior Fatah official Qadura Fares, who is a member of the Prisoner's Club - the main organization representing Palestinian prisoners - said, "Today we declare on behalf of the prisoners the launching of a prisoners' intifada, and we will not relent until the prisoners, who are a symbol of liberty and dignity, are released."
The Palestinian Liberation Organization issued a statement expressing its commitment to the release of all Palestinian prisoners. The PLO said any future agreement with Israel would be conditioned on their release.
According to the PA, some 315 prisoners currently held in Israel were detained prior to the Oslo Accords. The PA said among the prisoners being held by Israel are 330 minors, 37 women and 15 members of parliament or ministers.
The Palestinians say that 7,000 of the 19,000 Palestinians who have been arrested since 2002 are still incarcerated. They said Israel has detained a total of 760,000 Palestinians since 1967.
TheHamas government in Gaza also marked Prisoners' Day. Prime Minister Ismail Haniyeh said, "We must complete the reconciliation (with Fatah) and engage in widespread resistance in the West Bank to help the Palestinian prisoners who are being held in Israel."
During a rally in central Gaza, senior Hamas figure Halil al-Haya called on the Palestinian organizations to do whatever is necessary in order to "liberate Palestine and the prisoners."
Al-Haya demanded that Egypt free the Hamas men it had detained "because they had not committed any crime against Egypt," and also called on the PA to free prisoners belonging to Hamas and Islamic Jihad.
AP and Raanan Ben-Zur contributed to the report