Around 2,000 people joined a rally in front of the offices of the International Committee of the Red Cross in Nablus, in the northern West Bank.
The demonstrators waved Palestinian flags, pictures of prisoners, and banners with slogans reading: “No to solitary confinement.”
They also handed over a letter calling on the Red Cross to intervene on the prisoners’ behalf.
Further south, several hundred Palestinians held a similar rally in the center of Ramallah.
Demonstrators had set up a protest tent on Saturday in Ramallah’s Yasser Arafat Square, formerly known as Clock Square. The group, calling themselves independent youth, accused politicians of neglecting the cause of prisoners.
In Gaza City, around 1,000 people demonstrated on Monday outside the local headquarters of the Red Cross.
Hamas leaders addressed the crowd, and pledged that Gilad Shalit, an Israeli soldier captured by Gaza-based militants in 2006, would not be freed until Palestinian prisoners were released.
“We assure our courageous prisoners that the day of liberation approaches and that Shalit will not taste freedom or see the light until our prisoners have been released safe and sound,” Hamas leader Ismail al-Ashqar said.
Hamas has sought to exchange Shalit for hundreds of Palestinian prisoners in Israeli jails, but mediated talks between the two sides have repeatedly failed, with Israel refusing to free Palestinians accused of carrying out anti-Israeli attacks.
On Sunday, prisoner support group Addameer said Palestinian detainees affiliated to Hamas and Fatah were joining the Popular Front prisoners who launched the strike, protesting the treatment of PFLP leader Ahmad Saadat, who has been held in isolation for three years.
Director of the Abu Jihad center for prisoners affairs at al-Quds university Fahd Abu al-Hajj said Sunday that 3,000 or so prisoners were taking part.
There was no immediate comment from the Israeli Prisons Authority of the number of prisoners who refused food last week, but by Monday, IPA spokeswoman Sivan Weizman said “160 prisoners” were observing a hunger strike.
Prisoners are demanding an end to solitary confinement and collective punishment, fines and what they say are provocative cell raids.
They are also calling for improvements to medical care and to be allowed to take university courses and receive books, newspapers and clothes.
Palestinian Authority Minister of Detainee Affairs Issa Qaraqe said prisoners wanted better visitation rights, noting that visits were limited to 30 minutes each month. Detainees were protesting the practice of chaining their hands and legs during family visits and meetings with lawyers, he added.
In June, Israeli Prime Minister Benjamin Netanyahu vowed to toughen conditions for Palestinians in Israeli jails, in an effort to pressure Hamas to release captured Israeli solider Gilad Shalit.