"We are all Khader Adnan," chanted crowds gathered in the Gaza Strip, together with activists from the main political parties who joined forces in a rare display of Palestinian unity.

Adnan, 33, has been refusing to eat since mid-December following his arrest in the occupied West Bank. The mathematics graduate is being held under so-called "administrative detention", which means Israel can detain him indefinitely without trial or charge.

The Islamic Jihad group, which advocates the destruction of the state of Israel, has said it will escalate the violence if Adnan dies, following reports that his health was deteriorating.

"We will pursue our Jihad and resistance. We will sail in the sea of blood and martyrdom until we land on the shore of pride and dignity," top Islamic Jihad leader Nafez Azzam said during a Friday sermon at Gaza's oldest Al-Omari mosque.

The Physicians for Human Rights group in Israel (PHR), which has been monitoring Adnan's condition in an Israeli hospital, said on Friday he was "in immediate danger of death" adding that he had suffered "significant muscular atrophy."

The Israeli army has said in a statement that Adnan was arrested "for activities that threaten regional security." It has not given further details. Adnan, who is married and whose wife is expecting their third child, owns a bakery and a fruit and vegetable shop in his West Bank village, Arabeh. He has served as a spokesman for the Islamic Jihad, who describe him as a local leader.

At least 5,000 people took to the streets of Gaza, waving a mix of black Jihad flags, the green flags of Islamist group Hamas and the yellow flags of the secular Fatah movement of Palestinian President Mahmoud Abbas.

Witnesses said hundreds had also demonstrated in the northern West Bank city of Jenin, where Adnan was initially arrested.

Palestinian officials said many other prisoners in Israeli jails had started hunger strikes in support of Adnan, including Hassan Salama, a senior armed commander of Hamas who is serving a life sentence for masterminding suicide bombings against Israelis.

Palestinian prisoners have regularly staged hunger strikes in attempt to gain better conditions or to denounce the Israeli occupation of the Palestinian Territories.

However, such protests usually end quickly. Officials said no one has persisted for as long as Adnan who has been given days to live.

Azzam, from the Islamic Jihad, accused Arab states and Western powers of ignoring Adnan's protest. "Shame on the nations of hundreds of millions (of Muslims) for the fact that Khader Adnan is still in prison," he said in his Friday sermon.

Hamas, which governs Gaza, said it was putting pressure on the Arab League and Egypt to secure Adnan's release. "The Palestinian people, with all their components and factions, will never abandon the hero prisoners, especially those who lead this hunger strike battle," said Ismail Haniyeh, Hamas's top authority in Gaza.

The PHR rights group said Adnan could die even if he breaks his fast. "There is a risk to his health even if he starts eating now because his system has got so used to not having any food at all," a spokesman confirmed.