The 45-year-old went on hunger strikes several times after previous arrests, including a 55-day strike in 2015 to protest his detention without charge.
A medic from the group Physicians for Human Rights Israel visited Adnan in prison this week and warned that he “faces imminent death” and called for him to be “urgently transferred to a hospital”.
The armed group Islamic Jihad warned Israel that it will “pay the price for this crime” as it called the father of nine a “martyr”.
The Israeli military said missiles fired from the Hamas-controlled Gaza Strip fell in open territory but caused no damage.
The International Committee of the Red Cross (ICRC) in Israel and the Occupied Territories expressed their condolences to Adnan’s family, calling for Israel to release his body.
The organisation had visited the detainee several times since his arrest in February to monitor his condition.
Al Jazeera’s Nida Ibrahim, reporting from Arraba in the occupied West Bank, described an atmosphere of sadness as Palestinians mourn the death of Khader Adnan.
“Palestinian are saying that Israel wants to show the world who has control on the life and death of the Palestinians,” Ibrahim said.
“Adnan was expecting to be arrested without charges – this is an Israeli policy that it uses against many Palestinians,” Ibrahim said, noting that there are 1,000 of them held without charges in Israeli prisons. “But he was later charged with provoking violence and being a member of a terrorist group as far as Israel is concerned as it considers Islamic Jihad to be a terrorist group,” she said.
“He was the first person to inspire many Palestinians to use individual hunger strikes as a way to demand freedom, particularly when it comes to be held without charges,” Ibrahim added.
The Lebanese armed group Hezbollah has called for all Palestinian prisoners in Israel to be freed following the death of Adnan.
The group lauded Adnan’s “long march full of steadfastness” and “resistance” against Israel’s “arbitrary practices”, decrying the years he spent in prison undergoing “abuse and terrorism”, according to a statement in Al Manar, the party’s press agency, on Tuesday.
“We offer the Palestinian people and the resistance movements, especially the brothers in the Islamic Jihad movement and the bodies, clubs and associations of prisoners’ affairs, our deepest condolences and blessings on the martyrdom of the heroic resistance fighter, the captive Sheikh Khader Adnan,” the group stated.
Ghassan al Khateeb, a professor of political science at Birzeit University, Ramallah, says Israel’s remarks on how Adnan died cannot be trusted.
“Even if their story is true, it does not deny their responsibility for his death. They arrested him for no reason and did not provide him with the necessary healthcare,” he told Al Jazeera. “Not handing over the martyr’s body to his family could increase the escalation in the Palestinian cities.”
“We have to shut down our towns, and protest, and do more than that. He [Adnan] starved for 86 days. People here are conditioned to think its normal. It’s exhausting but we need to come out and do this, because no one is listening, people need to hear.”
Adnan’s wife Randa says she and her family do not want any response such as rocket attacks in light of her husband’s death.
“My late husband used to say not a single drop of blood should be spilled. Now after his death and all the sacrifices he made, we do not want a drop of blood to be spilled,” she told reporters outside their home in the occupied West Bank.
“We do not want to see any response for his death. We do not wish to see rockets fired and, in return, Gaza being bombarded.”