Hamas forces in the Gaza Strip committed serious human rights abuses including abductions, torture and extra-judicial killings of Palestinian civilians in 2014, a report says.
Most of the victims were accused of collaborating with Israel, Amnesty International investigators report.
The report says no-one had been brought to account for the abuses, suggesting they were officially sanctioned.
Hamas said the report was biased and had relied on dubious sources.
The report, entitled Strangling Necks, covered the period of last summer's 50-day war between Israel and militants in Gaza.
It says while "Israeli forces were inflicting destruction and death on the people in Gaza, Hamas forces took the opportunity to ruthlessly settle scores".
Hamas spokesman Fawzi Barhoum told BBC Arabic the report was "highly politicised and lacks professionalism and credibility.
"It wasn't based on accounts from all parties nor did it listen to Hamas or even the police force in the Gaza Strip. It was just based on accounts from some sources, which is not enough to write a report of such calibre from such an international rights organisation."
The fighting between Israel and the militants left at least 2,189 Palestinians dead, including more than 1,486 civilians, and 11,000 injured, according to the United Nations. On the Israeli side, 67 soldiers and six civilians were killed, with scores more wounded.
Amnesty has been deeply critical of the actions of both Israel and Palestinian militants in Gaza in recent reports, accusing both sides of abuses during the conflict. Israel and the Palestinians have rejected the allegations.
'Committed with impunity'
In the latest report, Amnesty says Hamas forces committed the abuses during Israel's military offensive, which began on 8 July and ended on 26 August 2014.
According to the report, at least 23 Palestinians were subjected to summary, extra-judicial executions. Most were already in prison, accused of acting for Israel, it says.
In addition, "perceived political opponents" of Hamas were abducted, tortured or assaulted - "particularly members of the rival Fatah party and former members of the Palestinian Authority security forces in Gaza".
"These abuses too were committed with impunity," the report says.
BBC Middle East correspondent Kevin Connelly says Hamas exercises undisputed authority within Gaza, and Amnesty's powerful report depicts an organisation responding to the relentless pressure of Israeli military operations with a brutal campaign against its own enemies within.
In one incident, six men were shot dead outside a mosque as they knelt in hoods in front of a crowd of men, women, and children, the report says.
The killings were among a series of "spine-chilling actions, some of which amounted to war crimes, [and] were designed to exact revenge and spread fear across the Gaza Strip," Amnesty regional director Philip Luther said.
Amnesty called on the Palestinian authorities to "impartially and independently" investigate the allegations, and bring the perpetrators to justice.