"Marwan would go out for small runs in the jail every morning with other prisoners and he was at his best physically and mentally before going on hunger strike," his lawyer and longtime friend Elias Sabbagh told Fairfax Media.

The strike, called by Barghouti to protest the conditions in which Palestinians are held and the use of administrative detention by Israeli military courts in the occupied territories, is now entering its fourth week. There are around 6500 Palestinian prisoners, with estimates of over 400 children detained.

Barghouti, 58, a key leader in the Fatah party who rose to prominence during the second Palestinian intifada, or uprising, is a more popular figure among Palestinians than his nominal superior, Palestinian Authority president Mahmoud Abbas, who recently met with US President Donald Trump.

But he has been imprisoned by Israel since 2002, serving five life sentences on charges of spearheading attacks during the second intifada that killed five people. Barghouti has rejected the legitimacy of the Israeli courts.

As recently as 2011, a report by Israel's military courts - which handle cases against Palestinians living in occupied territory - revealed that the conviction rate for accused Palestinians was 99.74 per cent.

"This is not a legal matter anymore. This is a national and political case that all Palestinians care about - their own prisoners," Sabbagh added.

Fadwa Barghouti, Marwan's wife, has not heard from him as visitation rights have been cut - a common occurrence for Palestinian families.

"There's no news from him. The only thing we received is a curt letter from the International Red Cross informing us that he sends us his best," she told Fairfax Media from Ramallah.

She sent a message to Pope Francis this week imploring him to talk about the suffering of political prisoners, including her husband.

Last week, the Israel Prison Service (IPS) released footage purporting to show that Barghouti had broken his salt-and-water hunger strike by secretly snacking. The footage was broadcast widely on Israeli media.

But when Pizza Hut tried to capitalise on the controversy to market its services, mockingly suggesting Barghouti could have ordered one of its pizzas, the backlash among Palestinians and across the Arab world forced the multinational to apologise and pull the ad.

A few days into the hunger strike, which started on April 17, a group of Israeli settlers held a barbecue outside Ofer prison in an effort to taunt and demoralise the prisoners.

Earlier this month, Israel's Channel 2 television network reported that the IPS was considering the use of foreign doctors to force-feed prisoners on hunger strike, as the Israeli Medical Association has consistently refused to allow its members to participate in such actions.

For years, international human rights groups as well as the United Nations have condemned arrests of Palestinians under military law and incidents of medical neglect, torture and isolation of prisoners. The conditions in Israeli prisons are particularly harsh on minors, according to experts.

"The law for Palestinian children is different than for Israeli children," said Mikko Joronen, a researcher at Tampere University in Finland who has studied the treatment of Palestinian children in detention.

"Stone throwing, for example, is considered in Israeli military law as a serious security violation which can carry a sentence of up to 20 years," he said.

The hunger strike comes as the 50th anniversary of Israel's occupation of the West Bank during the 1967 Arab-Israeli war approaches.

Donald Trump is scheduled to visit Israel on May 22, but hopes that the standoff with the hunger strikers will be over by then are not high.

Sabbagh's hope is that the plight of the prisoners moves the international community to put legal pressure on Israel.

"Ultimately there's a major demand from the Palestinian people after 50 years of occupation - it is time to free the Palestinian people and release their prisoners," he said.

"The prisoners are living in tight isolated jail cells and the Palestinian people are living in an open-air prison whether in [the] West Bank or Gaza or Jerusalem."