(CNN) -- Security forces under Hamas rule have tortured Gaza prisoners during the group's five years of rule, a new report said Wednesday.
The Human Rights Watch report also documents "serious abuses" in Gaza's criminal justice system, such as arbitrary arrests, incommunicado detentions and unfair trials in the Palestinian territory of Gaza since the Hamas political movement took control in 2007.
"After five years of Hamas rule in Gaza, its criminal justice system reeks of injustice, routinely violates detainees' rights, and grants impunity to abusive security services," Joe Stork, deputy Middle East director for Human Rights Watch, said in an online posting Wednesday. "Hamas should stop the kinds of abuses that Egyptians, Syrians, and others in the region have risked their lives to bring to an end."
In a bullet-point statement on its website, Hamas' Interior Ministry disputed the report's accuracy and called it biased and political.
Titled "Abusive System: Criminal Justice in Gaza," the 43-page report said "there is ample evidence that Hamas security services are torturing people in custody with impunity and denying prisoners their rights."
The group said Hamas has executed at least three men "convicted on the basis of 'confessions' apparently obtained under torture."
Human Rights Watch says the rivalry between the Palestinian movements of Hamas and of Fatah, which prevails in the West Bank, "remains a significant factor" behind many of the alleged Hamas detainee abuses
The group said the Fatah-dominated Palestinian Authority "arrests and detains Palestinians arbitrarily, including Hamas members or sympathizers, and similarly subjects detainees to torture and abuse."
Human Rights Watch's findings are also consistent with increasing reports of abuse by security forces in Gaza against detainees accused of nonpolitical crimes, including people accused of drug offenses and fraud.
Some of the cases documented have been "against people detained on suspicion of collaborating with Israel or the Palestinian Authority in the West Bank. Collaboration is a serious crime under Palestinian law, but suspicion of collaboration does not justify torture or other abuse."
Abdel Karim Shrair, for example, was charged with collaborating with Israel, the report said.
His family and lawyer said he was tortured. But the group said military courts didn't "adequately address Shrair's claims of torture."
Last year, a firing squad executed Shrair. According to the report, "his mother said that Hamas authorities had prohibited the family from burying him, and that police beat her when she tried to hold his body during the interment."
"There is ample evidence that Hamas security services are torturing people in custody with impunity and denying prisoners their rights," Stork said. "The Gaza authorities should stop ignoring the abuse and ensure that the justice system respects Palestinians' rights."
Islam Shawran, a spokesman for Hamas' Interior Ministry, said Hamas gave Human Rights Watch all of the information it requested, but was surprised that it wasn't taken into consideration.
And why, he asks, is the reporting about incidents from 2007 and 2008 now being released?
"It is obvious because it is a cover for the Zionist occupation. Also this report did not discuss the basic human needs of the Gaza people by discussing the Israeli siege on the Gaza Strip and did not indicate to any of the violations committed by the Israeli Zionist enemy to our Palestinian prisoners inside the Israeli jails and prisons," he said.
CNN's Kareem Khadder in Jerusalem contributed to this report.