The jailed Palestinian leader Marwan Barghouti is at immediate risk in Israeli jails, where he has been attacked three times in as many weeks, including in one assault last month where prison guards set a dog on the 66-year-old, his lawyer has said.

Barghouti is often called Palestine’s Nelson Mandela. He is respected across otherwise feuding Palestinian factions, has broad popular support across occupied Palestine, repeatedly engaged with Israeli officials before his detention and long backed a two-state solution.

On Wednesday, the 24th anniversary of his imprisonment, international celebrities including Cate Blanchett, Bryan Adams and Don Cheadle added their names to a high-profile demand for his release that is already backed by hundreds of cultural figures and former global leaders.

His lawyer, Ben Marmarelli, said in a statement after a prison visit where Barghouti provided details of the most recent attacks that the Palestinian leader faced a “clear pattern of escalating abuse: violence, medical neglect, and treatment that places him at immediate risk” in jail.

Marmarelli said that on 25 March prison guards entered Barghouti’s cell in Megiddo prison with a dog and forced him to the ground where “the dog repeatedly attacked him”. The following day Barghouti was assaulted during a transfer to Ganot prison, and on 8 April guards at that jail beat him severely, left him bleeding for more than two hours and denied requests for medical treatment, he said.

“Despite all of that, his mind was sharp, focused, and deeply engaged with everything happening outside those prison walls,” Marmarelli said.

Barghouti’s trial for murder was criticised as flawed by legal experts, and Mandela himself reportedly described it as a legal attack on a legitimate political struggle. Barghouti was convicted of ordering attacks that killed civilians during the second intifada and has remained in jail since, spending long stretches in solitary confinement.

Mandela, South Africa’s first democratically elected president after the end of apartheid, in 2002 told his lawyer Khader Shkirat: “What is happening to Barghouti is exactly the same as what happened to me. The government tried to de-legitimise the African National Congress and its armed struggle by putting me on trial.”

Israel has rejected repeated requests for Barghouti’s release during prisoner exchanges, and recent years have brought growing concerns about his health, given widespread, systemic torture and abuse of Palestinians held in Israeli jails.

Prisoners do not get enough to eat, despite repeated orders from Israel’s supreme court to increase food provision.

“In an atmosphere where Israel is emboldened in its abuse of prisoners the only way to protect Marwan is to secure his immediate release,” Marmarelli said, adding that the visit took place under what he described as “absurd conditions”.

The two men were divided by glass and without working phones, so were forced to shout to make themselves heard in their conversation. “While Marwan is held in an Israeli jail there can be no way to guarantee his safety,” he said.

The Palestinian leader was seen in public footage for the first time in a decade last summer when the far-right national security minister, Itamar Ben-Gvir released a video of himself taunting a gaunt-looking Barghouti in jail.

His son, Arab Barghouti, later said the family feared for his life, after receiving reports that he had been beaten unconscious by eight Israeli prison guards. Family visits to the jail are barred, but former detainees released under a ceasefire deal provided evidence of the attack.

Arab Barghouti thanked the artists speaking up for his father and called for others to join the campaign.

“Artists and musicians were critical in the movement to free Mandela and end apartheid in South Africa and I hope they can play a similar role for Marwan and Palestine,” he said. “More than ever we need these voices to focus attention and turn hope into action.”

In Bethlehem Palestinian artists marked the anniversary of Barghouti’s detention with a mural on the Israeli-built separation wall.

The Israel Prison Service did not respond to requests for comment on Barghouti’s treatment.