The European Union on Saturday called on Israel to improve conditions for Palestinians in its jails, and a Palestinian minister said there would be rallies next week to support hunger-striking prisoners.
Nearly 5,000 Palestinians are currently held in Israeli jails, many charged with involvement in attacks on Israelis.
EU foreign policy chief Catherine Ashton said in a statement that she was "following with concern" the deteriorating health of four Palestinian inmates who are staging long-term hunger strikes in protest against their detention.
"The EU calls on the government of Israel to allow for the immediate restoration of their family visiting rights and calls for the full respect of international human rights obligations toward all Palestinian detainees and prisoners," she said.
A spokeswoman for Israel's Prison Service said the four prisoners were in "good condition" and receiving medical treatment as needed, but they had lost the right to family visits when they began their protests.
Issa Qaraqea, minister for prisoner affairs in the West Bank-based Palestinian Authority, said public demonstrations to show solidarity with the group would be held all next week.
On Friday, Palestinian protesters clashed with Israeli soldiers at a rally outside an Israeli prison in the West Bank.
Of the prisoners Ashton listed, Samer al-Issawi has been on hunger strike the longest, for 208 days, and has been treated a number of times in hospital.
Israel had released Issawi from jail in 2011, along with more than a thousand other prisoners as part of a swap to free captured soldier Gilad Schalit. But Israel arrested Issawi again in January 2012, saying he had violated the conditions for his release by engaging in terrorist activity.