Marwan Barghouti's health has declined over the past week after the leading Palestinian political figure joined a hunger strike with 1,100 other prisoners in Israel, an NGO said.

Barghouti is refusing medical treatment causing his health to decline seriously, Amani Sarahneh, of the Palestinian Prisoners' Club, told AFP news agency.

Barghouti is 57 and serving five life sentences over his role in the second Intifada,

The hunger strike began on April 17, with those taking part ingesting only water and salt. They have issued demands ranging from better medical care to phone access.

READ MORE: Palestinian prisoners launch mass hunger strike

An Israeli Prison Service spokesman denied that a decline in his health condition had been observed.

The Palestinian Committee of Prisoners' Affairs released a statement on Monday saying that Israeli Prison Service officials had forcibly moved hunger-striking prisoners to different sections of Israeli jails, confiscated clothes and personal belongings and placed leading figures in solitary confinement.

According to the NGO, Israeli authorities at the Jalame prison, where Barghouti was transferred and placed in solitary confinement after the strike began, have pressured him to accept medical treatment and urged other prisoners to try to convince him.

Some 6,500 Palestinians are currently held by Israel for a range of offences and alleged crimes.

About 500 are being held under Israel's system of administrative detention, which allows for imprisonment without charge.

Palestinian prisoners have mounted repeated hunger strikes, but rarely on such a scale.

Barghouti is popular among Palestinians, with polls suggesting that he could win the Palestinian presidency.

He was convicted of attacks during the second Intifada of 2000-2005 that killed five people, though he declined to defend himself and did not recognise the court's legitimacy.

Symbol of Israeli occupation

For Palestinians, the prisons have become a stark symbol of Israel's occupation.

Some 850,000 Palestinians have been imprisoned since the start of Israel's occupation 50 years ago, Palestinian leaders say.

Palestinian leaders have denounced Israel's refusal to negotiate with the hunger strikers, warning of a "new Intifada" if any of them die.

Demonstrations have been held in the occupied West Bank and Gaza Strip to support the prisoners, occasionally resulting in clashes with Israeli forces, though they have been limited in number.

A Palestinian former minister, Hamas member Wasfi Kabha, released on the fourth day of the hunger strike, said on Facebook that Barghouti had told him "the strike aims to achieve more humane, fairer and more dignified prison conditions".