Amnesty International said in a report today that Hamas committed war crimes against Palestinian civilians in Gaza during last year's war with Israel.
A ceasefire last August ended 50 days of fighting between Gaza militants and Israel.
Health officials said more than 2,100 Palestinians, mostly civilians, were killed in the conflict.
Israel put the number of its dead at 67 soldiers and six civilians.
"Hamas forces carried out a brutal campaign of abductions, torture and unlawful killings against Palestinians accused of' collaborating' with Israel and others during Israel's military offensive against Gaza," the human rights group's report said.
A Hamas spokesman criticised the report as being unfair, unprofessional and not credible.
In a previous report in March, Amnesty also criticised Israel and accused it of war crimes during the conflict.
Apart from the many deaths, at least 16,245 homes were destroyed or rendered uninhabitable.
Gaza militants fired thousands of rockets and mortar bombs at Israel.
In today’s report, Amnesty listed a number of cases it described as "spine chilling" in which Palestinians accused by Hamas of helping Israel were tortured and killed.
Hamas controls Gaza, a coastal enclave on the Mediterranean which borders Israel and Egypt.