Palestinian lawmakers probing the death in custody of a preacher from the Islamist Hamas movement said on Thursday he had been "tortured to death" by security men loyal to President Mahmoud Abbas.

A self-appointed investigation team comprising six independent lawmakers determined that Majd al-Barghouthi died as a result of torture and said the head of the Fatah-run intelligence services, Tawfiq Tirawi, should be held to account.

Another commission which Abbas appointed in late February to investigate the matter has not yet delivered its verdict, although a pathologist working on behalf of Fatah said he had not found signs of torture on Barghouthi's body.

Independent lawmaker Hassan Khreisheh, a member of the investigating commission, said it had observed "torture marks" on the legs, back and arms of the 45-year-old father of nine, who had been detained for a week before his death.

He said witnesses had told the commission that Barghouthi had been tortured in a "mad manner".

"We demand that this incident close the chapter of political detention once and for all," Khreisheh told Reuters.

He said the lawmakers had demanded that Abbas punish anyone who participated in, ordered or oversaw the torture of Barghouthi.

In February, Hamas officials accused security agents from Abbas's secular Fatah faction of torturing Barghouthi to death but security officials said he had died of a stroke.

Tension between Hamas and Fatah has increased since the Islamists took control of the Gaza Strip in a brief but bloody civil war in June. Fatah still holds sway in the Israeli-occupied West Bank.

Human rights groups accused Hamas of torturing to death four Palestinians in Gaza since it took control of the Strip. Both Fatah and Hamas accuse each other of arresting and torturing their supporters in the enclaves.