One detainee told of being beaten with pipes and having a screwdriver rammed into his back. Another said interrogators tied his hands behind his back then lifted him into the air by his bound wrists.

Two human rights groups on Monday decried widespread torture of political opponents by bitter Palestinian rivals Hamas and Fatah, and Associated Press interviews with three victims and a doctor backed the reports of abuse.

The findings emerged as the two sides carried out fresh arrest sweeps in the West Bank and Gaza — highlighting deep tensions in the Palestinian territories after a flare-up in violence over the weekend.

In the West Bank on Monday, the security forces of Palestinian President Mahmoud Abbas rounded up more than 50 suspected Hamas supporters, including mosque preachers and intellectuals, in retaliation for a similar sweep of Fatah loyalists in Gaza, set off by a bombing that killed five Hamas members Friday.

Hamas violently seized power in Gaza in June 2007, leaving the Islamic militant group in charge of the coastal territory and Abbas' forces controlling the West Bank.

The Palestinian human rights group Al Haq said Monday that arbitrary arrests of political opponents have been common since Hamas' takeover of Gaza, with each side trying to defend its turf.

"Arrests for political reasons haven't stopped for a second," Al Haq director Shawan Jabarin told reporters. He estimated that before the latest sweeps, more than 1,000 people had been seized by each side.

An estimated 20 to 30 percent of the detainees suffered torture, including severe beatings and being tied up in painful positions, said Jabarin, citing sworn statements from 150 detainees.

He said three died in detention in Gaza and one in the West Bank.

"The use of torture is dramatically up," added Fred Abrahams, a senior researcher for Human Rights Watch, a U.S.-based group that is releasing its own report on abuse this week.

Jabarin said that while he had no proof of an official torture policy, he believed political leaders were indirectly encouraging abuse by looking the other way.

Abbas' prime minister, Salam Fayyad, acknowledged "shortcomings," but said human rights violations have 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," Fayyad said.

In Gaza, Hamas spokesman Fawzi Barhoum acknowledged "mistakes" were made by the Hamas forces, but said that unlike in the West Bank, violators were increasingly being punished. He also accused the Fayyad government of trying to destroy Hamas in the West Bank with U.S. backing.

Human Rights Watch said Abbas' forces need to come under closer scrutiny because of the international support they enjoy. Funding of Abbas' forces should be linked to an improvement in the human rights record, Human Rights Watch said.

Two branches of the Palestinian security, the national forces and the civil police, receive training from the U.S. and Europe, respectively. Neither force was cited in the Al Haq report as being abusive, and in both cases, human rights training is part of the curriculum.

"The Palestinians themselves are looking to restructure the security force into a more accountable, transparent force," said Colin Smith, who leads the European effort.

The U.S. State Department said it had not seen the reports. "However, claims such as this obviously concern us greatly," said spokesman Gonzalo Gallegos. "This is why it's so important to establish a situation where we can provide for the security of all Palestinians."

On the streets, spiraling Hamas-Fatah tensions are setting the tone. The latest round began Friday evening, when a car bomb killed five Hamas members and a 6-year-old girl in Gaza City. Hamas blamed Fatah, which denied involvement, and rounded up some 200 Fatah supporters.

On Monday, Fatah struck back. Abbas' forces set up roadblocks across the West Bank city of Nablus, checking motorists' names against lists of wanted people. Intellectual Abdel Sattar Qassem, a frequent Abbas critic, was taken from his home, his family said.

Nathera al-Qouni stood outside Nablus' Jneid prison, waiting to hand clothes to her 35-year-old son Mustafa al-Qouni, who was arrested at a checkpoint. "He is not Hamas, he is just a mosque preacher," she said.

Al Haq described methods used by interrogators in both territories. Commonly, detainees' heads are covered by sacks and their hands tied behind their backs. They are made to stand for hours. Those who move risked beatings on arms, legs and the soles of feet. Other methods included threats, humiliation and isolation in tiny cells.

Three ex-detainees — two from the West Bank village of Salem and one from Gaza — gave similar accounts to the AP.

Jabour, a 33-year-old construction worker, said he was detained on Nov. 17 by military intelligence in Nablus, near Salem. He said he was asked where he had hidden the automatic rifle of his late brother, a member of the Hamas military wing killed by Israel in 2002.

Jabour insisted he had no ties to Hamas and did not know of a weapon.

He said that for the next six days, he was beaten severely with sticks, pipes and fists, including on the soles of his feet. His legs became so swollen and his feet so sore that he couldn't stand, he said.

Jabour said he was taken to Nablus' Rafidiyeh Hospital after an interrogator rammed a screwdriver into his back, making him pass out.

Dr. Marwan Jayousi, who examined Jabour, told the AP that his legs were heavily bruised and very swollen. "There were a lot of marks of beatings by sticks, on his back, on his scapula, shoulders, and it was painful," the physician said.

Jabour was released without charge several days later.

Hosni Jabara, 50, also from Salem, said he was arrested by the Preventive Security Service in Nablus on Jan. 28, and was tied up in painful positions off and on for 32 days.

At times, he was pulled off the ground by a rope hanging from the ceiling and attached to his hands tied behind his back, Jabara said. He said he told interrogators he's a proud member of Hamas, but that he has no knowledge of weapons, and he eventually was released.

In Gaza, a Fatah supporter said he was beaten severely by Hamas agents for several hours, until he lost consciousness and had blood streaming down his face. After initially agreeing to be quoted and photographed, he withdrew permission, saying he had received new threats from Hamas.

Associated Press writers Ali Daraghmeh in Nablus, Mohammed Daraghmeh in Ramallah and Ibrahim Barzak in Gaza City contributed to this report.

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