Hamas is preventing human rights lawyers from visiting dozens of Palestinians arrested as part of the Islamist group's crackdown on rival factions in the Gaza Strip, a Palestinian rights group said.

Hamas began the crackdown after a July 25 bombing that killed five of its members and a girl.

Dozens were arrested after the attack, which the Islamist group blamed on Palestinian President Mahmoud Abbas's Fatah faction. Fatah denied the charge.

The Gaza-based Palestinian Centre for Human Rights (PCHR) yesterday described the detentions as illegal and said some of the arrests had been carried out by Hamas's armed wing, the Iz el-Deen al-Qassam Brigades, without warrants.

Raji Sourani, the director of the PCHR, told Reuters by telephone from Gaza City that his lawyers had been prevented from visiting any of those detained by Hamas for political reasons since the crackdown began on July 26.

Hamas spokesman Sami Abu Zuhri said today the PCHR report was "unfair and unbalanced" since it ignored arrest campaigns by Abbas's forces against Hamas activists in the occupied West Bank during the same period.

In an apparent response to Hamas's crackdown, pro-Abbas security forces rounded up scores of Hamas activists and officials in the West Bank.

The PCHR, citing testimony from victims, said Hamas was abusing and torturing some detainees. Last month, Palestinian rights group al-Haq said Fatah and Hamas were both guilty of failing to curb abuse and torture by their security men.

Tensions have been high between the rival factions since Hamas routed Fatah forces and seized control of the Gaza Strip in June 2007. Abbas's Fatah still holds sway in the West Bank.