Some 3,000 Palestinians in Israeli jails on Wednesday refused breakfast as part of a one-day hunger strike to mark Prisoners' Day, an Israeli prison official said.
Palestinians across the territories are attending marches and rallies as a show of solidarity with prisoners from the West Bank, east Jerusalem and Gaza held in Israeli facilities, whose numbers according to Israeli rights group B'Tselem reach 4,713.
In the West Bank, around 600 relatives of prisoners gathered for a sit-in in the rain at Arafat Square in central Ramallah after which some of them were planning to march towards the nearby military prison at Ofer.
Another rally was being held in the northern city of Nablus.
Meanwhile in Gaza, hundreds of people were taking part in a march from central Gaza City to the offices of the International Committee of the Red Cross, an AFP correspondent said.
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Another sit-in was to take place at Damascus Gate in east Jerusalem at 1400 GMT, officials said.
Events to mark Prisoners' Day began on Tuesday when youngsters in Gaza City released thousands of balloons into the air, each bearing the name of a prisoner, while there were also solidarity gatherings in Rafah in the south.
During the evening, demonstrators gathered in a village near the southern city of Hebron lit a "freedom torch" in front of the home of Arafat Jaradat, a prisoner who died in Israeli custody last month after several days of interrogation, sparking Palestinian allegations of torture.
"It was a symbolic event to deliver a clear message, firstly to the Israeli side and secondly to the international community, that it is intolerable for us to continue receiving our prisoners as corpses," Amjad al-Najjar, head of the Hebron branch of the Prisoners' Club, told Voice of Palestine radio.
Also during the evening, there were candle-lighting ceremonies in Gaza City as well as in Hebron, AFP correspondents said.
Of the Palestinians currently detained, some 169 are being held under administrative detention orders, whereby they are held without charge for renewable six-month periods, according to the B'Tselem human rights group.
Last year's Prisoners' Day was symbolic because it was the day when Israel released Khader Adnan, an Islamic Jihad inmate who went on hunger strike for a record 66 days in protest at being held without charge.