Hamas admits its forces in Gaza have made "mistakes" but it is cracking down
Human rights groups have accused rival Palestinian security forces in the West Bank and Gaza of making politically motivated arrests and torture.
Al-Haq, a Palestinian human rights group, said torturing detainees was widespread in the Hamas-controlled Gaza Strip and the West Bank under Fatah.
Both the Islamist Hamas and nationalist Fatah have acknowledged failings but say the problem is being dealt with.
They have been bitterly divided since Hamas seized power in Gaza in 2007.
Al-Haq's 85-page report, based on 150 sworn statements, listed various forms of torture and abuse applied by both sides to Palestinian detainees belonging to opposing factions.
The group estimated between 20%- 30% of detainees suffered torture, including severe beatings and being tied up in painful positions, said al-Haq director Shawan Jabarin.
"The use of torture and ill-treatment against detainees has clearly become a widespread trend," the report said.
The report "unequivocally concludes that the majority of arrests in the West Bank and Gaza Strip are politically motivated".
Al-Haq said such mistreatment had led to three deaths in Gaza and one in the West Bank since June 2007.
US-based Human Rights Watch is releasing its own report on alleged abuses by Fatah and Hamas this week.
Speaking ahead of its release, senior researcher for the group Fred Abrahams told AP news agency: "The use of torture is dramatically up."
Palestinian Authority Prime Minister, Salam Fayyad, acknowledged "shortcomings," but said human rights violations had decreased.
"I'm not defending anyone, but I can assure you that we have treated flaws and don't allow violations. The upcoming reports will be better," he said.
In Gaza, Hamas spokesman Fawzi Barhoum acknowledged that "mistakes" had been made by Hamas forces, but said violators were increasingly being punished.
The reports come after Palestinian security forces in the West Bank have detained more than 50 Hamas members, following a crackdown by Hamas in Gaza.
Academics, students and local council members are said to be among those picked up in and around Nablus on Monday.