Human Rights Watch on Wednesday accused Gaza's ruling Hamas movement of "extensive" rights violations, including torture, arbitrary arrests without warrants, and unfair trials.
But the Islamist movement hit back, calling the report "completely political," and denying that there was torture in its detention facilities.
The New York-based watchdog's report, entitled "Abusive System: Criminal Justice in Gaza," alleges that Hamas security services also failed to inform relatives of the whereabouts of detainees, and had arrested and abused lawyers.
"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," HRW's deputy Middle East director Joe Stork said in a statement.
"Hamas should stop the kinds of abuses that Egyptians, Syrians and others in the region have risked their lives to bring to an end."
HRW said it had interviewed victims of torture and other abuses, along with their relatives and lawyers, as well as judges and local rights groups.
"Witnesses reported that the Internal Security agency, the drugs unit of the civil police force, and police detectives all torture detainees," the group said, adding that one local rights group received 147 complaints of torture by the three forces in 2011 alone.
The report also criticised what it called "unfair trials," condemning the use of military courts to try civilians, and noted that several people had been executed after military trials resulted in death sentences, despite credible evidence they had been tortured.
"The military judiciary did not throw out any criminal cases against detainees because of due process violations, and ignored or failed to investigate credibly detainees' claims they had been tortured," the group said.
Three criminal defence lawyers told Human Rights Watch that they had also been detained, with two saying they had been "abused or tortured," the report said.
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The group acknowledged that Hamas had allowed the heads of three Palestinian rights organisations "ad hoc" access to detainees, but called on the government to expand that access.
It also noted that Hamas had reportedly disciplined some police officers accused of abuses, but that it had not made public details of the punishments.
Hamas criticised the report, denying that torture was widespread.
"This report is completely political and everything in it is based on conjecture," Hamas interior ministry spokesman Islam Shahwan told AFP.
He said the Hamas government took the report seriously, and noted that the ministry had disciplined 120 members of the security forces in 2011 for various infractions.
"We categorically deny that there is any torture in detention and affirmed that by opening these facilities to human rights centres," he added.
Rights groups and activists have long accused Hamas of various rights violations, charges the government rejects. It accuses the rival West Bank government of president Mahmud Abbas of mistreating Hamas members there.
Human Rights Watch acknowledged that the West Bank government is also responsible for rights abuses.