Relatives held up pictures of prisoners and waved Palestinian flags during separate demonstrations in Ramallah and Nablus in the occupied West Bank.
"Even if the whole world submits, we will not recognise Israel," chanted the protesters in Ramallah.
Thousands of Palestinians have been detained in Gaza, the West Bank and Israel since the start of the Israel-Hamas war in October, the United Nations human rights office said this week.
They have mostly been held in secret and in some cases subjected to treatment that may amount to torture, the OHCHR said in a report.
"For 10 months, we haven't known anything about our sons," Latifa Abu Hamid, a mother of four prisoners, all sentenced to life, told AFP.
"We want to check on them and see them. We want to know their situation... We want our sons."
According to the Prisoners Club, a Palestinian watchdog, about 9,700 Palestinians are currently in Israeli jails, including hundreds under administrative detention.
The NGO estimates that arrests have doubled since October 7 compared to the same period last year.
The OHCHR report said that since the October 7 Hamas attacks, thousands of Palestinians -- including medics, patients, residents and captured fighters -- have been taken from Gaza to Israel, "usually shackled and blindfolded".
"They have generally been held in secret, without being given a reason for their detention, access to a lawyer or effective judicial review," this week's OHCHR report said.
Testimonies for the report suggested that Israel had subjected prisoners to "a range of appalling acts, such as waterboarding and the release of dogs on detainees", UN rights chief Volker Turk said.
Israel did not comment but it has rejected previous critical reports, saying its prisons are run according to international law.
The UN report was released the day after Israeli military police questioned soldiers arrested on suspicion of sexually abusing a Palestinian detainee.
At the Ramallah demonstration, Umm Abdullah Hamed detailed how her brother, son and nephew had all been given multi-decade sentences.
"We feel like any family of a prisoner," Umm Abdullah Hamed, whose brother, son and nephew have all been sentenced to decades in prison, said at the Ramallah protest.
"We ask God Almighty to hasten their relief and freedom, God willing," she added.
The October 7 Hamas attack on southern Israel resulted in the deaths of 1,197 people, mostly civilians, according to an AFP tally based on Israeli official figures.
Militants also seized 251 hostages, 111 of whom are still held captive in Gaza, including 39 the military says are dead.
Israel's retaliatory military campaign in Gaza has killed more than 39,550 people, according to the Hamas-run territory's health ministry, which does not provide details on civilian and militant deaths.
