A Palestinian prisoner died in an Israeli jail on Saturday, sparking violence in the West Bank city of Hebron, his home town, and a call from the Territories' prime minister for a full inquiry.
The Palestinian Prisoners' Club meanwhile said the number of inmates on hunger strike had climbed to 11.
Israel Prisons Service spokeswoman Sivan Weizman said prisoner Arafat Jaradat, a 30-year-old father of two, had died suddenly at Megiddo detention centre in northern Israel.
"It was probably a cardiac arrest. I don't have additional details at the moment," Weizman said.
A statement from the office of Palestinian prime minister Salam Fayyad said he "expresses his deep sorrow and shock over the martyrdom of prisoner Arafat Jaradat in Israeli occupation prisons".
He "affirms the need to promptly disclose the true reasons that led to his martyrdom", it added.
After news of Jaradat's death, clashes broke out in Hebron between Palestinian youths and Israeli security forces, witnesses and security sources said.
The Islamist Hamas rulers of the Gaza Strip said the prisoner had died because of "the inhuman conditions in Israeli jails".
Israel's Shin Bet internal intelligence service said Jaradat had been arrested on Monday for his involvement in a stonethrowing incident in November 2012 during which an Israeli had been wounded.
"After lunch, as he was resting in Megiddo prison, Arafat Jaradat was taken ill. Medics were called to treat him but they were unable to save his life," it added.
The intelligence service said Jaradat had back aches and had previous injuries, one to the left leg from rubber bullets and one to the stomach from a tear gas canister.
He had been examined several times by a doctor, "who did not find any medical problem. The investigation continued", Shin Bet said.
Israeli police were now probing Jaradat's death, it added.
Israeli rights group B'Tselem said the investigation should look at how he was treated during interrogation and what techniques had been used.
It should be "independent, effective and transparent, and be finalised swiftly", the statement added.
Israeli media reported, meanwhile, that security officials had invited the Palestinian Authority to be present on Sunday for his autopsy.
Issa Qaraqaa, the Palestinian minister in charge of prisoner affairs, said a Palestinian doctor and Jaradat family members would be present.
Relations between Israel and the Palestinian territories are already particularly tense because of protests this month in solidarity with four other prisoners detained by Israel who are on hunger strike.
The Ramallah-based Prisoners' Club announced that another seven prisoners had joined the protest action, although there was no immediate confirmation from the prisons service.
"The Prisoners' Club does not know exactly when they started their hunger strike. The lawyers who are going to visit them will get more details," spokesman Amani Sarahna told AFP.
He said Muna Qaadan, a sister of Tareq Qaadan who has gone without food on and off for several months, was among those who joined the action. She had been freed in October 2011 under a prisoner exchange but since re-arrested.
An Israeli official said three hunger strikers -- Qaadan, Jafar Ezzedine and Ayman Sharawrna -- were taken to hospitals for protective check-ups on Friday.
The death of the prisoner Jaradat came a day after nearly 100 Palestinians were wounded in clashes with Israeli security forces during demonstrations in the West Bank to demand the release of the hunger strikers.
The fate of Palestinian prisoners, and in particular the hunger strikers, has sparked protests across the Palestinian territories and raised international concern.